Influence of Fatigue on Jump and Land Movement Patterns

Influence of Fatigue on Jump and Land Movement Patterns
St. Catherine University
SOPHIA
Doctor of Physical Therapy Research Papers
Physical Therapy
4-2016
Influence of Fatigue on Jump and Land Movement
Patterns
Sarah Bard
St. Catherine University
Beth Anne Cooper
St. Catherine University
Kevin Kosel
St. Catherine University
Owen Runion
St. Catherine University
Kristi Thorwick
St. Catherine University
Follow this and additional works at: http://sophia.stkate.edu/dpt_papers
Recommended Citation
Bard, Sarah; Cooper, Beth Anne; Kosel, Kevin; Runion, Owen; and Thorwick, Kristi, "Influence of Fatigue on Jump and Land
Movement Patterns" (2016). Doctor of Physical Therapy Research Papers. Paper 48.
This Research Project is brought to you for free and open access by the Physical Therapy at SOPHIA. It has been accepted for inclusion in Doctor of
Physical Therapy Research Papers by an authorized administrator of SOPHIA. For more information, please contact ejasch@stkate.edu.
INFLUENCE OF FATIGUE ON JUMP AND LAND MOVEMENT PATTERNS
by
Sarah Bard
Beth Anne Cooper
Kevin Kosel
Owen Runion
Kristi Thorwick
Doctor of Physical Therapy Program
St. Catherine University
April 1, 2016
Research Advisor: Professor Jaynie Bjornaraa, PT, PhD, MPH, SCS, ATR, CSCS
ABSTRACT
BACKGROUND AND PURPOSE: Injuries to the anterior cruciate ligament (ACL) are
common among both male and female athletes. Both the female gender and fatigue have
been demonstrated to increase injury rates. This research aims to reconcile the movement
pattern and fatigue protocol with what is seen in sport, while including both men and
women to see differences between knee biomechanics. The research will compare the
lower extremity biomechanics of a jump-land (double and single leg) between healthy
men and women after a sports specific fatigue protocol. Ultimately, this research is
intended to examine movement patterns which may predispose the subject to ACL injury.
METHODS: Twenty healthy subjects were studied (26.3 ± 3.5 years old), 10 of which
were female. A 3D electromagnetic system measured knee kinematics and kinetics during
3 jumping tasks. The subjects completed 4 sets of 3 different, randomized jumps
(bilateral to bilateral, bilateral to single-leg right, and bilateral to single-leg left) on force
plates. The subjects completed a fatigue protocol consisting of jumping, sprinting, stepups, and an agility ladder and were immediately re-assessed by completing the 3 different
jumps. A paired t-test was used to analyze pre and post fatigue and a one-way ANOVA
was used for analyzing gender comparisons for each variable.
RESULTS: Significant differences were found between pre-fatigue and post-fatigue
internal rotation and adduction knee angles for all 3 landings; other angles and knee
moments were significantly different dependent on type of landing. When comparing
i
gender for each variable, internal adduction moments and ground reaction forces were
significantly different for all landings. Knee angles were also significantly different
dependent on type of landing and dependent variable. Finally, females demonstrated
greater biomechanical changes in landing mechanics post-fatigue than males.
CONCLUSION: The results support previous literature that fatigue and gender have an
impact on jump and land movement patterns at the knee. The differences in knee angles
and moments from the current study, as seen by internal adduction moments and ground
reaction forces, demonstrate that fatigue and the female gender are risk factors for ACL
injury. This may support the current pattern of greater ACL injuries in female athletes,
especially when doing a jump-land movement.
ii
The undersigned certify that they have read, and recommended approval of the research
project entitled:
INFLUENCE OF FATIGUE ON JUMP AND LAND MOVEMENT PATTERNS
Submitted by,
Sarah Bard
Beth Anne Cooper
Kevin Kosel
Owen Runion
Kristi Thorwick
in partial fulfillment of the requirements for the Doctor of Physical Therapy Program
Primary Advisor __
__ Date _April 26, 2016_
iii
ACKNOWLEDGEMENTS
We would like to acknowledge our research advisor, Dr. Jaynie Bjornaraa, for her
support and guidance on this project, as well as her many hours spent dedicated to this
project. We would like to thank all of our research subjects who were willing to take the
time to participate in the research study. We would also like to thank our classmates,
friends, and family for their encouragement. Lastly, we would like to thank George and
Junior for their support throughout this project.
iv
TABLE OF CONTENTS
Chapter I: Introduction
1
Chapter II: Review of Related Literature
6
Chapter III: Materials and Methods
47
Chapter IV: Results
56
Chapter V: Discussion
62
Chapter VI: Conclusion
69
References
70
v
1
CHAPTER I
INTRODUCTION
Injuries to the anterior cruciate ligament (ACL) are common; a 2007 metaanalysis estimated that annually over 200,000 injuries occur in the United
States.1 According to a study looking at the societal and economic impact of ACL tears,
these new injuries and their long term effects annually cost society between eight billion
and eighteen billion dollars.2 Seventy percent of these incidences occur via a non-contact
mechanism; often as a result of increased dynamic valgus and internal or external rotation
of the knee. In regards to contact injuries, the mechanism often involves contact with a
valgus stress while the foot remains planted.3
Females are at a higher risk for tearing the ACL. Depending upon the sport the
individual participates in, females have approximately four to nine times the injury rate of
males. Anatomical and structural factors, hormonal factors, and neuromuscular and
biomechanical factors have been suggested as reasons for the increased incidence among
females.4 Chandrashekar et al.5 found that having a smaller intercondylar notch angle is
predictive of ACL tear for women. The same study also found that women have smaller
ACLs in length, cross-sectional area, and volume. A number of studies have suggested
that women tend to have greater ligamentous laxity because of less collagen, resulting in
failure at lower loads.5,6 Several studies have also demonstrated the influence of a
woman’s hormones on ligament properties. These studies have found that ligament laxity
and collagen strength vary throughout a woman’s menstrual cycle.7,8 It has been proposed
that the higher rate of ACL tears among women may be related to poor dynamic
2
muscular control. It is common to see increased dynamic valgus and increased loading of
the ACL, secondary to hip and core muscle weakness in women.4 Women also tend to
display more aberrant muscle recruitment or firing patterns, displaying increased or more
rapid activation of the quadriceps muscles in comparison to the hamstrings.4 Females also
demonstrate reduced hip and knee flexion angles upon landing; this results in increased
ground reaction forces at impact and increased stress on the ACL.4,9
In the United States 90% of patients who tear their ACLs go on to have surgical
reconstruction.10,11 The purpose of the reconstruction is two-fold: one, by recreating the
ligament through autograft or allograft, restoration of the knees biomechanical control
over anterior tibial translation is achieved, thus reducing the shear forces on the knee and
hopefully slowing the progression of osteoarthritic changes over time and secondly, to
restore function to the individual allowing them to return to an active lifestyle. Using the
Multicenter Orthopaedic Outcome Network database, Wright et al.12 found a 3% graft
rate failure and a 3% rate of tears in the contralateral knee occur during the first two years
after ACL repair. Five years after surgery, Salmon et al.13 found graft rupture to occur
within 6% of the population and contralateral ACL rupture to occur in another 6% of the
population. Shelbourne et al.ref further looked at the differences between men and
women as to which knee becomes injured after surgery based specifically on age, activity
level, and time to return to sport. Similar to Salmon et al. 13, the study found that within
five years 5.3% of participants had suffered an ACL injury to the contralateral knee and
4.3% had suffered an ACL injury to the repaired knee. In further analysis, Shelbourne et
al.ref found that women suffered more injuries to the contralateral leg than men but not
3
more injuries to the reconstructed knee. The report further details that younger patients
were at a higher risk for a second ACL injury (17% for those younger than 18) as
compared to older patients (4% for those older than 25).14 One study by Paterno et al.15
places the rate of reinjury as high as 27%.Patients post-ACLR see a marked decrease in
proprioception as is evident in a study by Delahunt et al.16, which showed a decreased
postural stability of individuals with ACLR as compared to healthy controls on the Star
Excursion Balance Test in the posterior medial and posterior lateral direction.
Likewise patient’s post-ACLR note adverse effects on their jumping and cutting
performance. This is linked, again, to problems with neuromuscular control, and
muscular strength, which translates into poor movement patterns that put athletes at risk
for tearing an ACL again with rates as high as 27%.14,15 Multiple studies support the
changes in vertical ground reaction forces (VGRFs) when compared to subjects’ healthy
knees.11,17 These risk factors are furthered confirmed in studies that examine single leg
tasks. Ortiz et al.18 examined single leg drop and hop tasks, finding that in lower
eccentrically demanding tasks, there was little difference in mechanics between involved
and uninvolved limbs, however when the eccentric demand increased greater dynamic
knee valgus and knee extension moments occurred.
All athletes reach a state of fatigue at some point. Extensive research has been
conducted to determine if fatigue has a direct impact on injury risk through analysis of
VGRFs, hip/knee/ankle kinematic changes, jump height, etc. Fatigue, in studies, is
mimicked in different ways. Some studies choose endurance tasks such as running until
failure is reached, others focus primarily on fatigue through muscular activity such as
4
squats, while some try a combination by simulating gameplay as closely as possible by
mixing endurance activities with plyometric movements to reach a full body fatigue
before measuring lower extremity (LE) movement patterns. Post fatigue research, with
healthy subjects, has observed decreased muscle activation of the quadriceps and
hamstrings,19 decreased jump height,20,21 and changes in knee kinematics.22,23 When
measuring patients post ACLR, fatigue has shown to decrease hip and knee flexion,
increase hip rotation, and postural sway in both healthy individuals and patients post
ACLR increasing the risk of graft tear.23,24
Purpose
This research aims to reconcile the movement pattern and fatigue protocol with
what is seen in sport, while also including both men and women, to note differences
between knee and hip kinematics and kinetics. The research will compare the lower
extremity kinematics and kinetics of a jump-land (double and single leg land) between
healthy men and women after a fatigue protocol. Ultimately, this research may identify
whether a dysfunctional or high risk movement pattern exists which may predispose the
subject (male and/or female) to possible knee or ACL injury.
Hypothesis
Our hypothesis is that all subjects will display negative changes in their
movement patterns. However, we expect that women, as compared to men, will exhibit
5
more aberrant lower extremity kinematics and kinetics upon fatigue, further increasing
their relative risk of ACL injury.
6
CHAPTER II
REVIEW OF RELATED LITERATURE
Gender Differences in Lower Extremity Biomechanics
The amount of literature available on biomechanical variances between men and
women at the knee is vast. Through a review of the literature, a few main points were
consistently seen in the results of many studies. A possible explanation for the
biomechanical sex differences could be that, on average, men typically have more lean
mass than females.25 Montgomery et al.25 conducted a study to explore sex differences in
the absorption of energy in the lower extremity when amounts of lean mass were
accounted for. They did this by calculating the landing height of a drop vertical jump test
from the amount of lean mass available in each subject to dissipate energy. Each
participant was assigned a specific drop height for their gender and amount of lean mass.
This was calculated in an attempt to equalize the relative task demands. Participants
received a fan-beam DXA scan to determine his or her body composition. Once the DXA
scan was completed, each subject was paired with an opposite sex participant with the
same BMI.
Subjects consisted of 35 male and female pairs.25 Participants were recruited from
NCAA Division I and club soccer and basketball athletes. Exclusion criteria consisted of:
suspected pregnancy, a BMI greater than 30 kg/m26, current lower extremity pain, and a
history of lower extremity injury. Before the drop vertical jump test, each participant
completed a dynamic warm-up. The testing consisted of five successful trials from the
7
assigned drop height. Female participants were tested at their assigned height as well as
the height of their male counterpart. This second testing height was used to compare
biomechanics when females were subjected to an exaggerated task difficulty. The
researchers did not see the biomechanical sex differences that they were expecting. The
only statistically significant finding in this study was that males absorbed more total
energy at the hip. Possible limitations include the use of highly trained female athletes
and a drop height that was not high enough to produce biomechanical differences
between the sexes.25 The researchers concluded that it is likely that the biomechanical
differences seen between the sexes arises from a factor other than relative task difficulty.
Olsen et al.3 analyzed 20 videos, over 12 seasons 1988-2000, of ACL injuries.
Three doctors and national team coaches were brought in to systematically analyze the
videos for common ACL mechanisms, along with 32 ACL-injured players to recall and
compare injury characteristics of the video analysis. Video was broken down into: player
position, playing phase, activity, push-off knee, takeoff knee, landing leg, ball handling,
contact with another player, disturbed by another player, balance, attention, speed, and
anything unusual about the play. Coaches broke down and analyzed the game aspects and
physicians analyzed knee kinematics (foot position at foot strike, knee position at foot
strike, when during the phase did the injury occur, movement direction at the time, and
weight distribution between the players legs).3
Statistical analysis was compared between the analyses of the different doctors
and coaches.3 Plant and cut was the main mechanism of injury followed by single leg
landing; in all the cases the foot was placed outside of the knee in a position of slight
8
flexion, valgus and external/internal rotation of the tibia. Sixty-six of 78 responses had
agreement, while 7 of the 12 disagreements were over contact or disturbance by an
opponent, and the last five were player position and activity at time of injury.3 These
findings are consistent with those supported in research, providing further bridging
between the lab and real world settings.
A study completed by Lyle et al.26 investigated the relationship between lower
extremity dexterity and differing movement patterns in females versus males. Lower
extremity dexterity is defined as: “the ability to dynamically control endpoint force
magnitude and direction.26” To quantify dexterity, the researchers used the lower
extremity dexterity test. This requires participants to compress an unstable spring with
their foot while attempting to attain the highest vertical force possible. While performing
the lower extremity dexterity test, participants were positioned such that all extremities
and trunk were supported excluding the lower extremity that was being tested. Five
practice tests were allowed before testing began. Each test consisted of a 16 second trial
in which the subject was instructed to “slowly compress the spring with their foot with
the goal to raise the force feedback line as high as possible and keep it there.26” Subjects
each completed between 21 and 25 trials with a 30 second rest between each test and a
two minute break between each fifth test.
This study also examined biomechanical landing differences between men and
women using a single-leg drop jump task.26 Each participant completed four trials in
which they were required to drop from a 30 cm platform, land on a single leg in the
middle of a force plate, and then perform a maximum vertical jump using their dominant
9
lower extremity. Three-dimensional kinematic data was obtained using an 11 camera
system, and ground reaction forces were calculated using the data from the force plate.
Subjects consisted of 14 male and female high school soccer athletes.26 Participants were
excluded from this study if they reported: history of ACL injury, history of knee surgery,
or an injury that inhibited their full participation in soccer for more than three weeks in
the last six months. The researchers discovered that females had decreased lower
extremity dexterity when compared to males. Lyle et al.26 further found that females had
increased leg stiffness, higher rates of ankle and knee coactivation, and earlier peak
vertical ground reaction forces in females than their male counterparts. One possible
explanation for these findings is that females exhibit an anticipatory stiffness reaction
before landing partially due to reduced lower extremity dexterity. The authors infer that
this could contribute to higher ACL injury rates in females, as well as explain why
exercise interventions have a positive impact on injury reduction. A limitation to this
study includes a small sample size, as the researchers only had 28 participants.26
Ali et al.27 authored a research article comparing total body mechanics of
recreationally active males and females. Participants completed single-leg landings with
their dominant lower extremity at various heights and distances. Leg dominance was
established by asking each subject which leg they used to kick a ball. Each subject
completed landings from a platform height of 20, 40, and 60 cm and were instructed to
jump horizontal distances consisting of 30, 50, and 70 cm. These different heights and
distances resulted in nine different landing conditions. Six male and six female subjects
participated in this study. Each subject was a recreational athlete, which was defined as:
10
“[participating in a] jump landing sport for 30 minutes a day at least 3 times a week.27”
Exclusion criteria consisted of any lower extremity injury at the time of data collection.
Before collecting data, each subject was given time to warm-up and practice single-leg
landings. Each participant was given enough time for them to feel comfortable with the
task. Participants were instructed to: “stand on their dominant leg, jump forward, and
land as naturally as possible with their dominant foot only centered on the force plate.27”
Furthermore, the subjects were instructed to: “keep their hands on their iliac crests when
landing to reduce any variability from swinging arms.27” Each participant completed two
single-leg landings from each of the nine landing configurations. One trial out of the two
completed was selected from each configuration from each participant for data analysis.
A seven camera system was utilized to capture data from 42 retro-reflective markers
placed on each subject. The force plate data was synchronized with the video
information.27
Through analysis of their collected data, the researchers found that higher trunk
flexion angles were associated with lower normalized peak vertical ground reaction
forces, and that higher knee flexion correlated with lower peak knee abduction.27
Furthermore, females had higher peak vertical ground reaction forces and a smaller ankle
plantar flexion angle when compared to their male counterparts. These findings suggest
that females generally have less ability to alter their ground reaction forces, increasing
their risk for non-contact ACL injuries. Additionally, the female participants’ limited
ankle plantar flexion angles resulted in a rearfoot landing strategy. This may also
11
contribute to their increased risk for ACL tears. Limitations include a small sample size
leading to an inability to generalize these finding to the larger population.
Schmitz et al.28 completed a study further examining sex differences in single-leg
landings. Each subject completed a 45 minute data collection session. This session
consisted of three to six practice jump-landings and five landings that were recorded for
further data analysis. Subjects were instructed to jump down from a 0.3 m platform and
land in the middle of a force place located 0.1 m away. These landings were completed
barefoot with the subject's’ dominant lower extremity. Researchers had their subjects
keep their hands on their iliac crests to reduce variability based on arm swing. Data was
obtained with the use of a force plate and six-degrees of freedom sensors attached to: “the
anterior mid-shaft of the third metatarsal, the mid-shaft of the medial tibia, the lateral
aspect of the mid-shaft of the femur, the sacrum, and over the C7 spinous process.28”
Participants consisted of 14 recreationally active men and women, which resulted in 28
total subjects.28 Recreationally active was defined as participation in any sport for at least
30 minutes three times per week or more. All participants were college students and did
not have a history of orthopedic surgery or neurological conditions affecting the lower
extremities.
Researchers found that females demonstrated a “stiff” landing style.28 Females
absorbed less total energy with their extensor muscles when normalized for body mass.
Athletes with a stiffer landing style have larger ground reaction forces. Landings with a
20% increase in ground reaction forces have demonstrated increased rates of ACL tear.28
The most limiting factor of this study, as reported by the authors, was that the landing
12
height was set at 0.3 meters for both genders despite males being taller and more massive
on average. This possibly increased the task difficulty for females compared to males.28
As sex differences in knee biomechanics continue to be observed in adult
populations, the question remained whether these sex differences are demonstrated in
children. Swartz et al.29 investigated the relationships between developmental stage, sex,
and biomechanics of jump landings. A maximum vertical jump height was determined
with the best of three attempts for each subject. After the maximum vertical height was
determined, a ball was suspended at 50% of their maximum vertical jump. Participants
were required to jump to the ball and land in a double leg stance. However, data was only
collected from the subject’s dominant limb. When landing, subjects were instructed to
land with both feet with their dominant foot in the middle of one force plate. A sixcamera system was used to collect three dimensional data from markers placed on the
dominant limb. Data collection occurred at two points during the landing: at initial
contact and at peak vertical ground reaction force. Measurements at these points included
flexion angles of the hip and knee, as well as knee valgus angles.
Participants consisted of 58 subjects without a history of back or lower extremity
injuries.29 Subjects were separated into groups based on their gender and developmental
stage. Girls from seven to ten and boys from eight to eleven were included in the
prepubescent developmental stage. These children were active in a youth sports program
that included jump landing activities. Adults from 19-29 were included in the postpubescent developmental stage. To be included in the study, adults were required to
participate in 30 minutes of activity at least three times per week. Exclusion criteria
13
included participation in an NCAA Division I jumping sport and the inability to complete
a mature vertical jump. A mature jump pattern was defined as having a “preparatory
crouch with 60° to 90° of knee flexion and a countermovement arm swing coordinated
with complete extension at the hips, knees, and ankles at takeoff.29” Results of this study
were inconclusive. The only statistical differences were seen in the biomechanics of
children versus adults.29 Children produced smaller hip and knee flexion angles while
demonstrating higher knee valgus angles. Sex differences in landing biomechanics were
not supported by this research. Limitations of this study include a task difficulty that was
possibly too low to elicit biomechanical differences between genders.29 More research
with larger sample sizes would be useful to clarify these inconsistencies in the literature.
In summary, females demonstrate stiffer landing styles than their male
counterparts. This single finding may have a profound impact on the ACL injury rates in
healthy males versus females. Stiffer landings typically equate to increased ground
reaction forces, which have been shown to increase rates of non-contact ACL tears.28
A study by Jacobs et al.30 evaluated the hip abductor function and lower extremity
landing kinematics of females and males. In this study there were 15 women and 15 men
that participated. The subjects were volunteers that were free from orthopedic injury
within the past 6 months. Ascension’s Flock of Birds protocol was used to acquire three
dimensional joint kinematics of the hip and knee, with sensors placed on the sacrum,
distal lateral thigh, and proximal lateral shank.30 The positioning of the subjects’ hip
joints were centered using the procedure by Leardini et al.31 Prior to testing, a 5-minute
warm-up was completed on the cycle ergometer.
14
The testing itself consisted of pre-fatigue functional landing trials, strength
testing, endurance testing, submaximal 30-second isometric exercise, and post-fatigue
landing trials. The functional landing tasks included a two-legged to one-legged forward
jump to a target area which was a distance of 40% of their height. Height of the jump was
also standardized as 15% of the subject’s height. Joint angles were taken upon initial
ground contact and peak joint displacement was calculated using joint data up to 500ms
following initial contact. Following the pre-exercise landing trials, hip abductor strength
and endurance tests were performed. Three 5-second maximal voluntary isometric
contractions (MVICs) of the hip abductors were completed with thirty second rest periods
in between. Strength and endurance were quantified using a dynamometer in sidelying.
For endurance, the subjects completed a submaximal isometric contraction of the hip
abductors until they were unable to hold the load for more than 3 consecutive seconds.
EMG activity of the hip abductors was collected during the endurance testing to ensure
fatigue of the hip abductors. The participants then had a 15-minute rest period followed
by a 30 second bout of isometric hip abduction followed by the post-exercise landing
trials. The results showed that overall women demonstrated higher valgus positions than
men during landing.30
Another study by Russell et al.32 examined frontal plane knee angles during initial
contact (IC) following a drop landing. There were 16 men and 16 women participants
who were free from lower extremity injury. The lower extremities were tracked with a
10-camera analysis system for both static and dynamic movements. A force plate was
also used to analyze landing ground reaction forces (GRF). Surface electrodes were
15
placed on the dominant limb to analyze EMG activity of the gluteus medius (GM). The
drop landing procedure consisted of the subject standing on a 60 cm box and dropping
onto the dominant limb in the force plate area on the floor below. In order for a trial to be
considered successful, the subject was required to stick the landing for at least 2 seconds
and to keep the opposite limb from touching the ground. Maximal knee flexion, initial
contact, and gluteus medius EMG activity were collected and analyzed. The results show
that females landed in valgus, while males landed in varus upon contact with the ground,
when taking into account only frontal plane motions.32 At maximal knee flexion angles,
males also reached a greater varus position than females.32
A study by Brown et al.33 used three-dimensional LE coordinates with anticipated
and unanticipated cutting. The participants included 13 men and 13 women who did not
have a history of serious knee injury, surgery, arthritis, or current knee pain. Leg
dominance of the participants was determined by subjectively asking which leg they
would use to kick a ball the farthest. The procedure of testing included single leg jump
landing tasks with a randomized cutting maneuver. The type of cut (left or right) was
determined by a colored light source. An anticipated cut was described by a light that was
shown prior to the participant jumping off of a block. A light shown as the jumper
crossed the boxes’ anterior plane simulated an unanticipated jump. If the participant was
shown a light to cut left, they would land on their right leg and cut immediately to the
left. Similarly, if the light shown was to cut right, the participant would land on their left
leg and cut to the right. The results showed that females presented with an increase in IC
hip flexion, adduction, and internal rotation when compared with men during anticipated
16
and unanticipated SL jump landings involving a cut.33 When comparing dominant and
non-dominant LEs, there was a significant increase in non-dominant hip and knee
internal rotation angles during peak stance when compared to the opposite limb. This
shows that there is potentially a difference between LE kinematics between the dominant
and nondominant LEs.33 These researchers suggest that there is a minimal increased risk
of ACL injury with the slight difference in LE kinematics and explain that there would
need to be further research to show increased disparities in leg dominance. It appears that
there may not be consistency in the research with regards to the definition of leg
dominance, which may be defined by one study as that leg which can kick a ball farthest
or the leg you stabilize on by another study. When comparing anticipated and
unanticipated jump cuts, there was a decrease in hip flexion during unanticipated cuts.33
This could potentially place a person at a greater risk of injury by putting the hamstrings
at a mechanical disadvantage to oppose anterior tibial shear loads.33 The decreased ability
of the muscles to stabilize the surrounding knee may increase the risk for injuring a
person’s ACL.33
Another study looking at both EMG activity of men and women was conducted
by Dwyer et al.34 This study consisted of 22 men and 22 women between the ages of 18
and 40. The participants were free from major lower extremity injury, LE surgery, and
were able to complete the three functional tasks being studied. Three-dimensional
kinematic data were collected at the knee and hip joints using the Ascension Flock of
Birds protocol as mentioned previously. EMG data was collected using a 16-lead system.
The skin on the muscle belly of the gluteus medius (GM) was shaved, cleansed, and
17
abraded to decrease impedance. The GM muscle activity of the dominant limb was
collected as described by Dwyer et al.34 The procedure used for examining the
participants included a single leg squat, lunge, and a step-up-and-over task which were
taught to the participants by one of the researchers prior to data collection. A 5 minute
warm-up was completed by each participant. The warm-up consisted of riding a bicycle,
followed by a LE flexibility program concentrating on the hip flexors, hamstrings,
quadriceps, and hip adductors. The electrodes were then placed on the skin and the
participants completed three MVICs for the above 5 muscle groups. MVICs were
performed 3 times each trial lasting 3 seconds, with a 30 minute break between each trial
and 2 minutes between each muscle group. Following MVICs of the muscle groups,
Flock of Bird’s protocol was used to determine resting hip, knee, and ankle joint
positions. The single-leg squat, lunge, and step-up-and-over exercises were performed
randomly 3 times each with 30 second rest breaks between trials and 2 minute rests
between exercises. The results of this study showed that women had smaller peak knee
flexion angles during all three tasks when compared to men.34 When looking at knee
valgus, there were no significant differences between men and women during all tasks.
Peak hip flexion angles in women were smaller than men during the single-leg squat
activity, whereas peak hip extension was greater in all tasks for females when compared
to males. There was no difference between hip adduction or hip external rotation during
any activities and when comparing genders.
18
Gender Differences in Electromyography (EMG) of the Lower Extremity
Women are at a much higher risk for tearing their anterior cruciate ligament
(ACL) in a noncontact situation than males and as a result, the comparison of
biomechanics and gender has been increasingly studied.30,32,35,36 Muscle activation, arc of
motion, kinematics, and torque are all variables that have been found to differ between
the two sexes.30,32,34-36 Electromyographic (EMG) activity of the lower extremity has
been studied previously during many closed kinetic chain (CKC) activities.32,34, 35 A
study by Youdas et al.35 was completed to determine EMG activity in the lower
extremities comparing men and women on both stable and labial surfaces. Fifteen male
and 15 female participants with no lower extremity pathology and a normal (grade 5)
MMT score of the LE were included. The subjects were recruited from a graduate school
physical therapy program and were all recreationally active. During testing, EMG
electrodes were placed on the muscle belly of the quadriceps and hamstring muscles
following an alcohol cleanse to reduce impedance. The ground electrode was placed on
the anteromedial aspect of the tibia. To get a base measurement the researchers manually
resisted the subjects’ knee extensors and flexors separately for 5 seconds to get a MVIC.
Order of labile or stable surface was determined randomly. The subjects completed a
single leg squat barefooted with their arms crossed over their chest while flexing their
trunk forward. The subjects squatted to a 45 degree angle using a 10 second cadence for
practice. During the testing portion, the extension phase of the movement was analyzed
for EMG activity. One single leg squat (SLS) on each limb was tested randomly with a
one minute rest period in between.35 EMG data was only taken from the 5 second
19
extension phase of the SLS. During the SLS activity, it was found that females have a
significantly higher activation of the quadriceps than males on both stable and labial
surfaces.34,35 In contrast, the results showed that males have a significantly higher EMG
muscle activation of the hamstrings than females, which supports the hypothesis that
females are quadriceps dominant and males are hamstring dominant.35 Since females tend
to be quadriceps dominant, this could create an increased shear of the anterior tibia on the
femur and a decreased ability to stabilize the knee during activity, which in turn could
place more stress on the ACL.35
A different study by Stern et al.36 looked at EMG activity, motor evoked
potentials (MEPs), and muscle strength of men and women pre and postfatigue. Participants included 17 males and 17 females that were recreationally active.
Subjects that had a concussion within the last 2 years, LE surgery in the past 6 months,
LE joint or muscle sprain within the last 6 weeks, or were unable to perform aerobic
activity were excluded from the study. EMG electrodes were placed over the muscle
belly of the vastus lateralis and biceps femoris. They were marked with permanent
marker to ensure their placement throughout testing. The subjects initially completed a 5
minute bike at a self-selected pace for a warm-up. Baseline measurements of MVICs of
the knee extensors and flexors were then taken using EMG. This was followed by a
functional exercise protocol to induce fatigue. The protocol consisted of treadmill
walking and body-weight-resisted exercise. The treadmill endurance began at a selfselected pace at 0% incline. Incline was increased by 0.5% each minute for 5 minutes.
Once completed, 10 alternating step-ups and 10 body squats were repeated until 1 minute
20
was finished. The total 6 minutes of exercise was repeated 5 times for a total combined
exercise of 30 minutes. Following each walking segment, rate of perceived exertion
(RPE) was taken. Measurements were taken immediately following exercise in the order
of MEP, EMG, and strength. This took no longer than 10 minutes. This study reported
that males had increased torque of the knee extensors and higher MEP amplitudes of the
hamstrings than females pre-exercise. Post-exercise, females exhibited decreased vastus
lateralis activation and increased knee-extension torque when compared to males, which
can indicate a difference in how each gender reacts to fatigue. The researchers suggest
that this could potentially create an increased risk of knee injuries in females, similar to
the previous study.36
Another study looking at EMG data was described previously.34 The results of the
study by Dwyer et al.34 showed that women were found to have increased gluteus
maximus activation during the three CKC exercises. This is hypothesized to be due to an
overall decrease in strength in women when compared to men, because when overall
strength is decreased the amount of activation needed for a task would be increased.34
During SL drop jump landing, the gluteus medius was found to have no differences in
activation between sexes.34 Although there was not a significant difference in healthy
individuals, the researchers suggest that in patients with lower extremity injury, there is a
possibility for an increased difference in gluteus medius activation as shown in other
studies. Therefore it is hypothesized that the gluteus medius could be responsible for knee
valgus during activity when a lower extremity injury has occurred.34
21
A study by Zazulak et al.9 looked at gender differences in EMG activity of hip
musculature during a single-leg landing. They specifically studied hip-stabilizing
muscles, including the gluteus maximus, gluteus medius and rectus femoris. Thirteen
female and 9 male Division I soccer and track athletes were recruited to perform drop
landings onto their dominant leg, as determined by participant report of which leg they
would use to kick a ball. The subjects performed 5 trials of single-leg landings from two
different box heights, 30.5 and 45.8 cm, and were told to “drop off the box, land on the
platform, and hold the position for at least 1 second”.9 Peak and mean EMG data were
recorded for two different time periods via surface electrodes. The 200 millisecond time
window before initial contact and the 250 millisecond time window after initial contact
were recorded. The EMG data were compared between gender using a 2 x 2 mixedmodel analysis of variance. The study reported significantly lower peak and mean
gluteus maximus activation in females compared to males after contact, and no difference
before contact.9 No differences were found between genders for gluteus medius muscle
activation before or after contact. Before contact, females had significantly greater peak
rectus femoris activation and no other differences were found in regards to the rectus
femoris activation. The authors suggested the decrease in gluteus maximus activation
may demonstrate that females may have a greater difficulty controlling the hip during
dynamic movement. This may lead to altered energy absorption during landing and
result in increased ground reaction forces believed to be associated with ACL
injury.9 Because imbalanced quadriceps contraction has been found to increase ACL
strain, Zazulak et al.9 also suggested that the increased activation of the rectus femoris
22
may contribute to increased ACL strain and other factors associating with ACL injury.
Therefore, the differences in EMG activity may play a role in the increased ACL injury
rates in females.
Effects Of Fatigue on Healthy Subjects
Many of the previous studies on biomechanics and kinematics have been
completed on subjects that have not had physical stress placed on them. Given the impact
prolonged activity or fatigue may have on knee movement patterns, it is important to
induce a state of physical stress on the body to better understand mechanics. When
attempting to better understand the reasons behind why ACL injuries occur, it is
important to get an understanding of what happens to the knee when fatigued as well,
since the majority of ACL injuries occur when a person’s body is physically tired. There
appears to be a moderate amount of information/research studies investigating the effects
of fatigue on jump-landing, muscle performance, and gender associated injury risk.
However, this information is greatly varied in terms of fatigue protocol, types of jumps,
and populations. Fatigue has resulted in many different changes in muscle activity, joint
kinematics, ground reaction forces and jump performance.20-22,38 Depending on the
change, an increased risk for injury, particularly ACL injury, may be present.37
Quammen et al.37 examined the effects of two different fatigue protocols on hip
and knee kinetics and kinematics: a slow linear oxidative fatigue protocol (SLO-FP) and
functional agility short-term fatigue protocol (FAST-FP). Power was calculated and
determined to be 15 participants with an alpha at .05; 15 DI female soccer players with
23
clearance from the team physician to practice were tested. Kinematics was examined via
8 high-speed infrared cameras and ground reaction forces were sampled through 2 force
plates. Only the dominant leg was sampled, and was determined as the leg the players
used to kick the ball the furthest. For the SLO-FP group VO2 levels were analyzed via a
metabolic cart.37
Running—stop—jump and side stepping were used to mimic
soccer moves and were randomly projected onto a screen in front of the players, which
they then had to execute at a minimum speed of 3.5 m/s to, again, mimic a real soccer
game. Five successful trials were completed followed by a 1-minute rest before their
fatigue protocol was initiated.
Prior to the FAST-FP, participants had their maximal vertical jump measured 3
times and then averaged and used for the protocol. The first activity was the step-up stepdown onto a 30cm box for 20 seconds at a 220 beats per minute pace (metronome to
pace); next participants completed an L drill via 3 cones spaced 4.5 yards apart from each
other and required participants to run down and back between cones 1 and 2, and then
back to cone 2 and to run around the left side of cone 3, back to cone 2 and to finish
sprinting to cone 1; participants then immediately completed 5 consecutive vertical jumps
of at least 80% of their maximal jump, which was marked on the wall; finally participants
completed a ladder drill with a metronome pacing them at 220 beats per minute—the first
time participants faced forward through the ladder drill, the second circuit they faced
their right moving laterally through the ladder drill and the last circuit they faced their
left. All these 4 drills completed once counted as 1 set. Four sets were consecutively
completed, which took about 5 minutes. The SLO-FP required participants to run at
24
9km/h for 5 minutes followed by a 1-km/h increase every 2 minutes until exhaustion.
Participants expressed exhaustion through grabbing the railings of the treadmill, at which
point the treadmill was slowed down. Maximal fatigue was noted by 2 of the following:
(1) heart rate reached 90% of calculated age-related norm, (2) respiratory quotient was
more than 1.1, (3) unable to continue running. Participants were then given a 5-minute
rest before completing a 30-minute treadmill run alternating between 2 speeds for 6
intervals: 4 minutes at 70% of the final VO2 max speed, then 1 minute at 90% of final
VO2 max speed. SLO-FP took approximately 45 minutes; 15 minutes for the VO2 max
and 30 for the intervals. Immediately following the fatigue protocol, participants
completed 5 successful randomized running—stop—jump tasks. FAST-FP participants
were required to complete 3 maximal vertical jumps to maintain fatigue between dynamic
trials.
Results, via a paired t-test, showed the FAST-FP group had a greater adduction
moment at initial contact (IC). Otherwise both fatigue protocols showed participants had
less hip flexion' post-fatigue than pre-fatigue at IC. At peak vertical ground reaction
force, participants had less hip flexion post-fatigue than pre-fatigue. At peak vertical
ground reaction force, participants had less knee flexion post-fatigue than pre-fatigue.
Despite the FAST-FP displaying more frontal plane displacement of the hip and knee
when compared to the SLO-FP, the authors concluded that both fatigue protocols
achieved an increased risk for non-contact ACL mechanisms being integrated into the
athlete’s motor outputs overall, thus increasing the risk of ACL tears.37
25
A study by Oliver et al.20 looked at the effects of a soccer-specific fatigue
protocol on jump performance and muscle activity. They measured jump height, GRF
data, and EMG activity. The subjects consisted of 10 youth male soccer players with an
average age of 15.8 years old. Three different types of jumps were included in this study:
a squat jump, a countermovement jump, and a drop jump. The squat jump was
performed from a starting squat position with knee flexed to about 90 degrees. The
countermovement jump started from an erect position and when cued a maximal vertical
height jump was performed. The drop jump consisted of dropping from a 35 cm height
followed by a maximal vertical height jump. The subjects performed 3 trials of each
jump with both feet on a single force plate and were done before and after the fatigue
protocol. The 42-minute soccer-specific fatigue protocol was completed on a nonmotorized treadmill. It consisted of 3 bouts of 14-minute exercise periods with 3 minutes
of recovery in between each bout. The 14-minute bout was made up of seven 2-minute
periods that included 45 seconds of walking, 15 seconds of cruising, 15 seconds of being
stationary, 40 seconds of jogging and 5 seconds of maximal sprinting. Jump height and
GRF data were recorded via the force plate and EMG activity was recorded using surface
electrodes. Paired t tests were conducted to analyze differences in jump performance
before and after the fatigue protocol, and a repeated-measures analysis of variance was
conducted to analyze differences in jump conditions. Oliver et al.20 found jump
performance, as determined by jump height, was significantly lower after the fatigue
protocol for all 3 jumps. There was no significant difference in jump performance
between each of the different jumps. The only significant difference in EMG activity due
26
to fatigue was during the drop jump. Specifically, they found significantly lower total
EMG activity and lower muscle activity in the vastus lateralis, biceps femoris, and tibialis
anterior, but not the soleus. When measuring impact, peak, mean, braking, and
propulsive GRFs, the only significant difference due to fatigue was found in the impact
GRF during the drop jump. This was significantly greater after the fatigue
protocol. Overall, there were changes seen in jump performance across all jumps with
fatigue, a decrease in muscle activity and an increase in impact force during the drop
jump after the fatigue protocol.20
Moran et al.38 investigated the effects of endurance fatigue and increasing heights
on drop jump performance and knee mechanics. They studied impact acceleration, knee
joint kinematics, and jump height before and after a fatigue protocol. Fifteen female
competitive soccer players who had at least 6 months of experience with drop jumps as
part of their training within the past 2 years were recruited. They performed 3 trials of
drop jumps from each of the 15, 30, and 45 cm heights before and after an endurancespecific fatigue protocol. The participants were instructed to jump vertically with
maximum effort while trying to spend as little time on the floor as possible.38 The fatigue
protocol consisted of running at a speed of 6 miles per hour with a 3% grade incline for
one minute before increasing the incline by 1.5% every minute afterward. The subjects’
rating of perceived exertion (RPE) was used to determine fatigue and was taken every 2
minutes during the protocol. On a scale of 6-20, an RPE of 17 or “very hard” was used to
determine when fatigue had been achieved. Impact accelerations were measured using an
accelerometer, knee motion was measured using an electrogoniometer, and jump height
27
was calculated using a foot switch apparatus. In order to compare the fatigued state to
the differing drop heights, a 2 x 3 within-subjects analysis of variance was
conducted. Moran et al.38 found significantly lower jump heights from all drop heights
with fatigue. Due to this finding of lower jump heights, the authors concluded that
neuromuscular fatigue was achieved with ratings of 17 or “very hard” on the RPE
scale. They observed tibial impact accelerations significantly increased with each
increasing height and they all were significantly larger with fatigue from heights of 15
and 30 cm, but not 45 cm. No significant changes were seen in knee flexion angles at
initial contact, peak knee flexion or the range of knee flexion. However, peak knee
angular velocity was found to be significantly larger when fatigued from heights of 15
and 30 cm, but not 45 cm. Peak knee angular velocity was also found to significantly
increase with each increasing height. Therefore, the authors concluded that caution
should be taken when performing drop jumps when fatigued due to the increased impact
accelerations and therefore, greater risk of injury.38
McLean et al.22 studied the effects of fatigue on hip, knee and ankle mechanics
during drop jumps. This was done via a fatigue protocol. They also compared results
between males vs females, and dominant vs nondominant legs within the study. Ten
male and 10 female Division I athletes, taken from basketball, soccer or volleyball, were
recruited as subjects. In order to be included in the study each participant needed to be
free of any past ACL injury or any current lower extremity injury that would prevent
them from participating in the drop jump or fatigue protocol. The drop jumps were
performed by stepping off of a 50 cm platform and, after landing, immediately jumping
28
vertically with the intent to achieve the maximum height possible and quickly as
possible.22 There were two separate force plates for each limb during landings. Ten trials
of the drop jumps were completed before and after the fatigue protocol. The fatigue
protocol lasted exactly 4 minutes and consisted of “a series of continuous drills that
loosely reflected tasks synonymous with actual game play”.22 The subjects were required
to perform 20 step-up and step-down movements as quickly as possible onto a 20 cm
step. Then they switched to plyometric bounding movements for a distance of 6 meters
before turning around and bounding 6 more meters to the starting point. The subjects
then went back to the step-up task and repeated this sequence as many times as possible
in 4 minutes. Kinetic and kinematic data were recorded using force plates and a threedimensional motion analysis system. To determine the effects of fatigue and gender, a
three-way mixed-design analysis of covariance was conducted. McLean et al.22 found
fatigue produced significant increases peak knee abduction and peak knee internal
rotation in both genders and legs. Fatigue also produced significant increases in peak
knee abduction and internal rotation moments in both genders. The authors cited
previous research that stated internal tibial rotation movement contributes directly to
ACL loading and, in turn, an increased risk of ACL injury. Therefore, their results imply
fatigue from their fatigue protocol increases ACL loading and risk of ACL injury. They
also found that females had significantly greater increases in peak knee abduction and
internal rotation, and had greater increases in peak knee abduction with fatigue compared
to males. The authors suggested that this difference may contribute to females’ increased
risk of ACL injury.22
29
A study by James et al.21 investigated the effects of two different fatigue protocols
on drop landing performance. They looked at kinetics, kinematics, and EMG activity
during the landings. Ten recreationally active males were recruited to be subjects and
were required to have no current lower extremity or spine injuries, or no past
surgeries. They completed 10 trials of the drop landings prior to either of the fatigue
protocols, and 5 trials were completed after each of the fatigue protocols. The drop
landings were done from a platform 61 cm high and each subject “initiated each landing
by slowly stepping out with their right foot, shifting their weight forward, then by quickly
bringing the left foot forwards and dropping straight down”.21 The subject's right foot
landed on a force plate with the left foot landing on the adjacent floor. Before any fatigue
protocol, subjects participated in an initial testing session in order to determine general
fitness and peak work rate on an ergometer. The first fatigue protocol consisted of
isometric squat contractions, while the other consisted of cycling. The isometric fatigue
protocol included repeated bouts of 15 second maximal isometric squat contractions
followed by 5 seconds of rest until the subject’s force output of their initial maximal
contraction dropped below 50% for more than half of a 15 second contraction. They
were then instructed to complete one additional 15 second maximal contraction. The
cycling protocol included cycling on an “ergometer at a self-selected pace between 60
and 80 revolutions per minute at an intensity equivalent to 60% of the peak work rate
achieved during the initial session”.21 Fatigue was achieved once the subject dropped
below 40 revolutions per minute despite encouragement. Each fatigue protocol was
completed 10 days to 6 months apart. GRFs were recorded via a force plate, knee
30
motions were measured using an electric goniometer, and EMG data were recorded using
surface electrodes. A 2 x 2 repeated measures analysis of variance was used to determine
the effects of fatigue and fatigue protocol. James et al.21 found significantly greater EMG
activity in the vastus lateralis and vastus medialis with fatigue. They concluded that this
supports the leg stiffness mechanism of fatigue which may contribute to ACL
injury. However, they found significantly lower GRFs at the second peak and total force
measures with fatigue, suggesting decreased leg stiffness. There was a trend towards
greater EMG activity with fatigue, but this was not significant. The authors also found
significantly greater knee flexion at contact after the isometric fatigue protocol compared
to the cycling protocol. The isometric protocol was meant to induce short duration
isolated fatigue, whereas the cycling protocol was aimed to bring about whole-body
fatigue that persisted into recovery. The knee flexion finding suggests that the fatigue
protocol has an impact on changes in kinematics and highlights the importance that
fatigue must be achieved in order to accurately study changes in kinematics.21
Although many ACL injuries occur during jumping and cutting, it is also well
known that these injuries can often happen while in the deceleration or eccentric portion
of a movement. Research was previously done by Zebis et al.19 to examine the
biomechanical and muscular effects of acute fatigue on the lower extremity (LE- hip,
knee, and ankle) of healthy subjects. In order to do this, Zebis et al.19 examined the
impact of acute fatigue on neuromuscular activity in female handball players. Prior to
fatigue participant’s maximal isometric voluntary contractions of the quadriceps and
hamstrings were taken, but the researchers noted this has little functional applicability to
31
ACL tears. Participants also had their quadriceps/hamstring EMG activity measured
while performing a side-cutting maneuver over a force plate, which the researchers
surmised would be more applicable to moments of ACL trauma. A fatigue protocol,
lasting 50 minutes, mimicking a handball match with various segmented tasks was
performed. Fatigue was based on heart rates collected from 4 handball athletes during a
match, which aimed to match those heart rates with their fatigue protocol. The fatigue
protocol consisted of low/high intensity running/sprinting with different levels of incline,
countermovement jumps (CMJ), single and double leg jumps, side-cutting, and
sidestepping. Post fatigue rate of perceived exertion (RPE) was measured at an average of
16±1, which rates “hard” to “very hard” on the 20 point Borg scale.19
Zebis et al.19 found that maximal voluntary contraction (MVC) of the quadriceps
and hamstrings decreased significantly, but did not have statistically different EMG
results when compared against one another. No statistical differences were found in knee
and hip joint angles or ground reaction forces (GRF); however, they did find significant
differences in neuromuscular activity in the pre-landing and landing phases of sidecutting (with p<.05). Specifically they noted decreased activity in the biceps femoris,
semitendinosus, and lateral gastrocnemius at 10 ms and 50 ms prior to landing. In the 10
ms after landing, biceps femoris and semitendinosus were again decreased, but 50 ms
after landing no statistically significant decreases were noted in the observed muscles
(vastus lateralis/medialis, rectus femoris, gluteus medius, biceps femoris, semitendinosus,
gastrocnemius lateralis/medialis).19
32
The findings show that ACL-agonist muscles experience a marked inhibition after
acute systemic fatigue. Zebis et al.19 hypothesize that the unexpected finding of reduced
sub-maximal neuromuscular activation of the hamstrings may be in part due to the
increased need to recruit more motor units to resist the quadriceps counter force, thus
being a totally different motor pattern and/or reduced synchronization of hamstring
activation. Likewise it could also be a purposeful decrease in hamstring activity to help
stabilize the knee joint angles and GRFs, which were not noticed to have altered in this
study. Although this may be advantageous for stabilizing knee angles and GRFs, it is not
an ACL protective strategy and has been shown to put the knee at greater risk for a
valgus moment or hyperextension in non-contact explosive moments like side-cutting,
jumping or landing. Limitations of this study include the fact that joint angles were not
measured with 3D kinematics.19
Another study looked at both EMG and MEPs (motor evoked potentials) of males
and females pre and post exercise.35 The exercise consisted of treadmill walking at
increasing incline, step-ups, and body squats. This study reported that males had
increased torque of the knee extensors and higher MEP amplitudes of the hamstrings than
females pre-exercise., Males exhibited greater vastus lateralis activation and a decreased
knee-extension torque when compared to females post-exercise. This suggests that there
is a potential loss of quadriceps torque after exercise, particularly in females, which can
indicate a difference in how each gender reacts to fatigue. The researchers suggest that
this could potentially create an increased risk of knee injuries in females, similar to the
previous study.35
33
Another study that compared males and females, as well as dancers to team-sport
athletes was completed by Liederbach et al.39 This study was conducted with the purpose
of comparing 40 dancers (20 males and 20 females) to 40-team sport athletes’ (20 males
and 20 females, D1-3) single-leg landing mechanics after going through a fatigue
protocol. The protocol consisted of 50 step-ups onto a 30 cm box (leading with right leg)
and 15 max effort vertical jumps. If vertical jump height decreased by 10% post fatigue,
landings were assessed via 3D kinematics and kinetics. The importance of comparing
dancers to team athletes was multifaceted: 1) dancers suffer less ACL injuries than team
athletes even though single-leg landing is common throughout performances and 2) there
is no sex disparity among ACL noncontact tears in dancers.39
Results showed that dancers took longer to reach a fatigued state, but no other
group interactions appeared statistically significant.39 A MANOVA showed female
dancers landed with a significantly lower knee valgus angle, hip adduction moment, and
trunk side flexion than female team sport athletes. However, there was no interaction
with fatigue and sex or group. Post hoc testing showed increased trunk flexion and lateral
trunk lean in a fatigued state of both the dancer group and athlete group. Quadriceps
dominance, defined as “preferential use of the quadriceps muscles to stiffen and stabilize
the knee joint during landings”, showed with group x fatigue, and main effects of group
and fatigue separately. Fatigue among both groups globally increased peak knee flexion
angles and decreased knee flexion moments.39
As shown above, proper knee biomechanics and kinematics cannot be assumed to
carry over post-fatigue even if they are exhibited in non-fatigued state. When an ACL
34
injury is involved we know improper and unnatural mechanics have occurred to stress
and strain the ligament to the point of rupture. Often times fatigue is a large contributor to
these faulty mechanics.
Lower Extremity Biomechanics of Subjects with Anterior Cruciate Ligament
Deficiency or Reconstruction
Injuries to the ACL are typically surgically repaired.11 In much of the literature it
appears that patients who have undergone ACL reconstruction (ACLR) continue to suffer
deficits. These include deficits of knee function, postural instability, multi-planar hip and
knee kinematic deficiencies, altered biomechanics with gait and dynamic landings, and
kinetic asymmetries. Studies by Paterno et al.11 and Miranda et al.17 found reduced peak
vertical ground reaction forces (VGRFs) during the landing phase of a drop vertical jump
(DVJ) and a jump cut maneuver for those who had undergone ACLR when compared to
the contralateral limb and to controls. In the Paterno et al.11 study 56 subjects who had a
unilateral ACLR, along with 42 healthy, activity-matched control participants, performed
a DVJ maneuver from a 31-cm box. The subject was instructed to drop off the box with
both feet leaving the box simultaneously and each foot landing on a separate force
platform. Because athletes frequently injure their ACL when landing from a jump, the
landing phase of the DVJ maneuver was used for analysis. Over three successful trials,
the mean vertical ground reaction force (VGRF) was found for each subject. A three way,
2x2 analysis of variance was used to compare the differences in VGRFs between the
involved and uninvolved leg of the group with ACLR, between the ACLR group and the
35
control group, and between the sexes. While there was no significant effect of sex noted,
the involved limb of the ACLR group showed significantly lower VGRF than their
uninvolved limb and both the preferred and non-preferred limb of the control group.11
The Miranda et al.17 study included 10 healthy subjects and 10 subjects who had
undergone ACLR at least five years prior. Each subject was asked to perform a jump-cut
maneuver. Standing one meter from a force plate, subjects jumped forward onto the force
plate after hearing the verbal command “Go”. At the same time of the verbal command, a
visual prompt for direction to the right or left was also given. In the direction of the visual
prompt, the subject performed a sidestep off of the force plate at a 45 degree angle.
Subjects had to perform 10 correctly executed trials. VGRFs were calculated based upon
the average of these 10 trials. Using two-way analysis of variance, comparisons between
gender and ACLR status were made. All subjects, regardless of sex, with ACLR landed
with decreased VGRFs.17 These findings are in support of other studies documenting
compensatory patterns of load transference and force absorption after ACLR.11,17
Delahunt et al.16 completed a study with 14 female athletes who had undergone
ACLR and 17 age- and sex-matched healthy controls. Each subject was required to
complete the International Knee Documentation Committee (IKDC) Subjective Knee
Form and the IKDC Subjective Knee Form, Knee Injury and Osteoarthritis Outcome
Score (KOOS). The authors reported a statistically significant difference, with large
effect sizes on the IKDC Subjective Knee Form, KOOSpain, KOOSsymptoms,
KOOSsport, and KOOSqol scales for those who had undergone ACLR. Each subject then
completed three trials of the anterior, posterior-medial, and posterior-lateral directional
36
components of the Star Excursion Balance Test (SEBT). The subject began the test by
standing barefoot on two adjacent force plates. Each trial was initiated when the subject
transitioned from double-leg to single-leg stance and ended when the subject returned to
the double-leg stance position. A 1.5m measuring tape was used to measure the reach
distances in each direction. Postural stability in the posterior medial and posterior lateral
directions on the SEBT were significantly impaired for those who had undergone ACL
reconstruction. These differences in postural instability did not hold true for the anterior
reach direction. Delahunt et al.16 further classified multiplanar hip and knee joint
kinematic deficiencies in their ACLR group when compared to a control group. These
deficiencies include greater hip adduction, less hip flexion, less knee flexion and
increased hip internal rotation with tasks of the SEBT.16 Similarly Webster and Feller40
found reduced knee flexion and less maximal internal rotation of the knee during single
limb hop and drop landings. In this study 35 ACLR patients and 13 healthy subjects
performed two functional landing tasks: a one legged horizontal hop and a one legged
vertical drop landing. The single leg hop consisted of a horizontal hop from a distance
equal to the subject’s leg length. The single-leg drop landing occurred from a 15 cm high
wooden platform. Three practice trials were performed on each leg for both tasks.
Significantly reduced knee flexion was documented for the ACLR patients and on the
contralateral/non-involved limb. For both landing tasks the patients also displayed less
maximal internal rotation .40 It is hypothesized that these significant alterations in joint
kinematics and loading patterns put individuals following ACL-reconstruction at a high
37
risk for reinjury of the ipsilateral or contralateral ACL. Rates of re-injury have been
reported as high as 27% in the literature.14,15
Another study by Paterno et al.41 found 4 variables combined to predict re-injury.
The study included 56 athletes who had undergone ACLR. Before return to sport each
subject underwent a 3-dimensional motion analysis during a DVJ maneuver and postural
stability assessment. These athletes were followed for an additional 12 months to record
occurrence of a second ACL injury. With a force plate under each foot, subjects were
asked to complete three successful trials of DVJ maneuver from a 31-cm box; VGRFs
and kinematic data were calculated for the landing phase of the DVJ. Each subject further
underwent single limb, dynamic postural stability assessed on both their involved and
uninvolved limb. Positioned on a single leg in the middle of a dynamic, unstable
platform, the subject was instructed to maintain a stable position for 20 seconds, 3 times
on each limb. The Balance System recorded movement of the platform away from a level
position. Three of the predictive biomechanical measures occurred during landing of the
DVJ task; these included: decreased hip external rotation in the uninvolved limb,
asymmetries in the sagittal plane knee moments of flexion, and increases in the frontal
plane knee joint range of motion of abduction. Subjects who displayed deficits in singleleg postural stability of the involved limb also demonstrated higher rates of second
injury.41
In contrast to most of the studies reviewed, Flanagan et al.42 found no deficits in
force production or reactive strength capabilities of those who had been well rehabilitated
(as determined by their score on the International Knee Committed Subjective Knee
38
Evaluation Form and with functional performance test) postoperatively. This study
utilized 10 athletes who had returned to sport following ACLR and 10 age- and activitymatched control subjects. Subjects completed three trials on each leg of two functional
performance tests: single-leg hop for distance and the 6-m timed hop. The participants’
best score for each leg in each test was used to calculate the leg symmetry index.
Utilizing a force sledge apparatus, each participant performed 4 testing protocols for
which the subject performed three jumps on each leg: the squat jump, countermovement
jump, drop jump, and rebound jump. For the squat jump, the participants started at a
position of 90 degrees of knee flexion; they were then instructed to “drive themselves
into the air” with maximal effort. The countermovement jump had the participant starting
in a position of full knee extension. The subjects were then instructed to jump as high as
possible. During the drop jump subjects were dropped from a height of .3m above the
force plate, along the sledge’s inclined track. Subjects were instructed to land with legs
toward extension and to jump rapidly off of the force plate. The final jump, the rebound
jump, was similar to the drop jump. Subjects were instructed to perform four maximal
jumps in quick succession after dropping from the same .3-m height. Statistical analysis
revealed no differences in the IKDC scores of the ACLR and control groups, as well as
no difference in leg symmetry index between the groups. The force sledge apparatus
testing revealed comparable degrees of between leg difference in both the ACLR and
control groups; however no differences between groups were noted.42
Another study by Ortiz et al.18 looked at differences of LE mechanics between
non-injured females and females with ACL reconstruction, as well as differences between
39
involved and noninvolved legs of females who have undergone ACL reconstruction.
They looked for any differences that occurred during a single-leg drop jump and a hop
task. Thirteen physically active females with ACL reconstruction were recruited. On
average, they were 7.2 years post-ACL reconstruction and ranged from 1 to 16 years
post-reconstruction. Fifteen healthy, non-injured females were recruited for the control
group. All subjects performed 5 trials of the single-leg drop jump from a height of 40
cm, and 2 trials of 10 single-leg up-down hops to and from a height of 20 cm. The drop
jump was done to mimic a land-and-go maneuver which is commonly performed in
sports.18 Leg dominance was determined based off of individual preference for a singleleg hop for distance. Knee and hip angles, knee moments, GRFs, and EMG data were
recorded via surface electrodes, motion analysis cameras and force plates.18 Three
different multivariate analyses were conducted to compare kinematics, kinetics and EMG
data between reconstructed and non-injured legs. Paired t tests with Bonferroni
correction were conducted on the same variables comparing involved and noninvolved
legs of subjects with ACL reconstruction. For the drop jump, the authors found no
differences in knee and hip angles between non-injured and reconstructed
legs.18 Significantly lower peak anterior-posterior shear forces were found in
reconstructed legs and significantly greater knee extension and valgus moments were
found in reconstructed legs. For up-down hops, no differences were found in knee and
hip angles, joint kinetics or EMG data.18 However, a significant difference was found
between involved and noninvolved legs in the ACL reconstruction group. Peak knee
extension moments were greater in noninvolved legs compared to involved legs. Ortiz et
40
al.18 have shown that women with ACL reconstruction demonstrate different lower limb
landing techniques when compared to the lower limbs of healthy, non-injured women
during a single-leg drop jump. However, in a task with lower eccentric and rotational
loads, the single-leg up-down hop task, those with ACL reconstruction demonstrate
similar landing techniques to those who have not been injured. They have also shown
that women with ACL reconstruction demonstrate closely symmetrical mechanics
between involved and noninvolved legs during a drop jump and up-down hop task.18
Lower Extremity EMG of Subjects with Anterior Cruciate Ligament Deficiency or
Reconstruction
As previously mentioned, a study recorded EMG data while looking at LE
biomechanical differences between non-injured females and females with ACL
reconstruction, as well as differences between involved and noninvolved legs of females
with ACL reconstruction.18 This study consisted of observing single-leg drop jumps and
an up-down hop task. The results showed significantly greater co-contraction ratios
between the quadriceps and hamstrings, greater gluteus maximus activation, and greater
rectus femoris activation in those who had ACL reconstruction compared to those who
did not. This suggests that women who had ACL reconstruction surgery may be
predisposed to future injury as their landing mechanics differ slightly from their noninjured counterparts. During the drop jump, there were no significant differences
between involved and noninvolved legs in subjects with ACL reconstruction. For the updown hop task, no differences were found relating to EMG data for either comparison.18
41
Effects of Fatigue on the Lower Extremity of Subjects with Anterior Cruciate
Ligament Reconstruction
Fatigue plays a dramatic role in biomechanical effects of the knee not only in
healthy subjects, but also in subjects following an ACL reconstructive surgery. Webster
et al.23 had 11 healthy men as control subjects and 15 men who had undergone primary
ACL reconstruction (ACLR) 15-19 months previously complete a general fatigue
protocol consisting of 10 body weight bilateral squats, 2 vertical jumps, 10 single-leg
drop landings (5 on each leg). This cycle was repeated 5 times or until fatigue—which
was stopped when jump height was reduced by 20%, or subjects could not complete the
protocol.23 Pre and post fatigue, subjects performed single-leg landings from a 30 cm
platform which were analyzed with 3D kinematics using 10 infrared cameras to see if
fatigue would affect landing mechanics, both among groups and also between surgical
leg and contralateral leg.23
The findings showed no significant differences between subjects with an ACLR
or the control subjects.23 Although, the ACLR group had a decrease in peak hip flexion
and ankle dorsiflexion as well as an increase in hip abduction and knee abduction and
internal rotation (IR) on the operative side when compared to the control group.
However, there were no significant interactions between fatigue level and group/limb for
any kinematic variable. Fatigue did impact the kinetic component of both groups;
decreases in knee flexion and adduction moments were noted, showing that individuals
are at a greater risk of tearing both ACLs regardless or ACLR history. Smaller knee
moments were noted in the ACLR limb compared to the contralateral limb as well as an
42
increased hip flexion moment on the operated limb compared to a decrease in the control
group. Webster et al. hypothesized that this was evident of compensatory strategies
occurring to help preserve lower limb stability.23
Frank et al.24 studied the effects of neuromuscular fatigue on landing mechanics
in active females with ACLR. The mechanics of fourteen physically active subjects
between the ages of 18 and 30 were assessed by 3D analysis when jumping off a 30 cm
box onto a force plate and then jumping as high as possible and landing on both legs.
Physical activity was defined as being active 3x/week for at least 30 minutes and rated on
the Marx scale (a self-report measure of frequency of cutting, running, jumping, etc.
activity). Mechanics were assessed by 3D analysis when jumping off a 30 cm box onto a
force plate and then jumping as high as possible and landing on both legs. Single-leg
balance was tested pre and post with eyes closed on a force plate. Fatigue protocol was
completed with squats from 0-60˚ at the rate of 25/minute with metronome pacing and
the barbell loaded at 30% of participant’s body weight. Participants went through 2
sequential squat cycles and rated their exhaustion on the Borg scale.24 Results showed
significantly decreased hip flexion, but no other statistical significances of kinematics at
the hip or knee were found.24 No kinetics were statistically different. Center of pressure,
as a measure of single-leg balance through sway speed (cm/s), significantly increased
post fatigue, which showed an increase in dynamic single leg stance. This study showed
that ACLR individuals are at an increased risk of injury when exposed to fatiguing
activity as was evident by less hip flexion at IC, which points to a reduction in muscular
43
resistance to fatigue and the neuromuscular system’s ability to sustain quality landing
patterns after fatigue.
As seen in many of the previous studies, there is a high amount of variability
between fatigue protocols, types of jumps, and recorded measures. It is often difficult to
see similarities in the presented research due to this inconsistency.
Landing Error Scoring System (LESS) and LESS-Real Time (LESS-RT)
Padua et al.43 created the Landing Error Scoring System (LESS) in order to
provide an “inexpensive clinical assessment tool … to provide a standardized instrument
… for identifying potentially high-risk movement patterns during a jump-landing
maneuver.”43 This assessment tool consists of 17 scored items, evaluating the positions of
the lower extremities and trunk at different times of the landing. The higher the score, the
worse the landing technique. The jump-landings are recorded using two cameras viewing
the sagittal plane and the frontal plane, and are later viewed for scoring. The jumplanding maneuver consists of a subject jumping forward off of a 30 cm box to a distance
of 50% of their height away from the box, and immediately rebounding for a maximal
vertical jump upon landing.43
Padua et al.43 recruited 2691 males and females from 3 different military
academies to be included in their study. At the time of testing, participants needed to be
healthy and free of orthopedic injury. They tested the concurrent validity of the LESS by
comparing it to the “gold standard” of a 2-camera three-dimensional motion analysis
system. 43 They also tested the interrater and intrarater reliability via intraclass correlation
44
coefficient (ICC). The authors found significant differences in biomechanics and groundreaction forces between those who scored high on the LESS and those who scored low on
the LESS. Those who scored high demonstrated biomechanics that have been shown to
be related to ACL loading and injury mechanisms. Therefore, the LESS was considered
to be valid with regards to assessing poor landing biomechanics during a jump-landing.
Interrater reliability was found to be “good”, ICC = 0.84. Intrarater reliability was found
to be “excellent”, ICC = 0.91. The authors also concluded the LESS was sensitive due to
standard error of measure (SEM) values below one. Low SEM values represent a low
estimate of standard deviation or variation in sample scores, meaning little variation in
LESS scores. The authors noted that although the LESS may assess poor landing
mechanics, it is uncertain if it predicts risk for ACL injury because this was not
investigated in this study. 43
Onate et al.44 tested the interrater reliability between expert and novice raters and
criterion validity of the LESS. They recruited 19 females who played soccer for a
Division I college to perform the LESS and be observed by a three-dimensional motion
analysis system. At the time of testing, all participants were free of low back or lower
extremity injury within the past 6 months or surgeries within the past 2 years. The two
raters of the LESS were both certified athletic trainers (ATCs). The expert rater of the
LESS had 15 years of experience as an ATC and 5 years of experience using the
LESS. The novice rater had less than 1 year of experience being an ATC and no
experience using the LESS, but received a one-hour training session. Overall, there was
“excellent” interrater reliability between expert and novice scores, ICC = 0.835. Within
45
nine different items of the LESS, Kappa values ranged from 0.459 – 1.0. Therefore, there
is excellent expert vs novice interrater reliability of the LESS. To assess validity of the
LESS, it was compared to a three-dimensional motion analysis system. If found to be
valid, the LESS would be able to accurately assess motion patterns similar to that of the
3-D system. Onate et al.44 concluded the LESS has a “moderate to excellent level of
validity,” but it is item dependent. Percent agreement was used to determine validity.
Ankle flexion at initial contact, knee flexion range of motion, trunk flexion at maximal
knee flexion, foot position at initial contact, and when stance width was greater than
shoulder width had “excellent” percent agreement. Trunk flexion at initial contact, when
stance width was less than shoulder width, knee valgus at initial contact, and knee valgus
range of motion had “moderate” percent agreement. Knee flexion at initial contact,
initial foot contact, and lateral trunk flexion at initial contact had “poor” percent
agreement. Onate et al.44 concluded that the LESS was “an ideally suited component of
any baseline examination for developing prevention programs associated with reducing
lower extremity injury” and that their “findings support moderate to excellent validity to
accurately assess three-dimensional kinematic motion patterns”.
Padua et al.45 created a different version of the LESS, the Landing Error Scoring
System – Real Time or LESS-RT. This was done in order for assessment of landings to
be done in real time instead of using cameras, making it more time efficient for
clinicians. With the jump-landing performed in the same way, the LESS-RT consists of
10 scored items over four trials of jump-landing, but it is observed and scored in real
time. The authors tested the interrater reliability of the LESS-RT. They recruited 43
46
healthy males and females free from any injury or illness to participate in their
study. Three raters evaluated the participants using the LESS-RT. All of the raters were
ATCs with over 5 years of experience and previous training and experience with the
LESS. The authors found ICC ranging from 0.72 – 0.81 and concluded “good” interrater
reliability. Padua et al.45 noted that they did not investigate the validity of the LESS-RT
and therefore it is unknown if the LESS-RT shares the same validity as the LESS.
47
CHAPTER III
MATERIALS AND METHODS
Subjects
A convenience sample of 20 healthy subjects volunteered for the study (26.3 ±3.7
years; 159.6 ± 27.3 pounds), 10 female (25 ± 2.7 years; 138.8 ± 17.9 pounds) and 10
male (27.6 ± 3.9 years; 180.4 ± 15.3 pounds). When the participants arrived, a consent
form was administered and signed by each individual prior to the assessment. A subject
intake form was then completed which included the participant’s age, activities, types of
competitive sports participated in, history of illness, and leg dominance. Leg dominance
was determined by asking which leg the participant would kick a ball with. The subject’s
height, weight, and waist circumference were also taken by the examiner. BMI was
calculated using the patient’s height and weight. The following exclusions prevented a
subject from participation: a recent history of orthopedic injury, a positive response to the
PAR-Q & You form, or if the participant did not consider themselves healthy enough to
participate given the demands of the tasks. This study had been approved by the St.
Catherine University Institutional Review Board prior to data collection.
Instrumentation
Three-dimensional joint kinematics were measured using Ascension’s Flock of
Birds electromagnetic motion capture system (Ascension Technology Corporation,
Burlington, VT) and Motion Monitor Software (Innovative Training Sports Inc., Chicago
48
IL). Two Bertec force plates (Bertec Corporation, Columbus, OH) placed side to side
were linked to the Motion Monitor system through an A/D interface panel (Measurement
Computing’s PCIM 1602 – 16 bit PCI board) for measurement of ground reaction forces
(GRF). Sensors measured 19.8 mm x 7.9 mm, which allowed precise placement over the
boney segments to be analyzed. The electromagnetic sensors were placed on the
participant’s sacrum, lateral thighs, and proximal lateral shanks. To control for interrater
error, only one researcher identified the placement of the sensors and marked the sensor
position with a pen so they could easily be placed on the lower extremity (LE) following
the fatigue protocol. Each sensor has an orthogonal axis system embedded within it and is
capable of an independent sampling rate of 100 Hz. The Flock of Birds system has a
reported static positional accuracy of 0.3 inch root mean-square (RMS) within a five foot
range from the transmitter and 0.6 in RMS within a 10 foot range. Static angular accuracy
is 0.5 RMS within a five foot range and 1.0 RMS within 10 feet (Ascension Technology
Corporation, Burlington, VT). The reliability and validity of electromagnetic motion
capture systems in gathering 3-D movements has been previously documented.28,46,47
Procedures
Kinematic Assessment
For kinematic assessment using the Motion Monitor integrated system
(Innovative Sports Training, Inc., Chicago, IL) with Ascension’s Position Capture
Technology, electromagnetic sensors were affixed to the skin with double-sided Velcro
49
and adhesive tape to sacral level two. Four other sensors were attached to each distal
lateral thigh near the iliotibial band (ITB) to avoid excessive movement, and to the mid
shank of each tibia. These sensors were secured with athletic pre-wrap and Velcro straps.
Anatomic bony landmarks on the pelvis, thigh, and shank were digitized for data
capture of lower extremity movement using International Society of Biomechanics (ISB)
recommendations for the hip and ankle and Grood and Suntay recommendations for the
knee.48,49,50 The Leardini method was used to determine the location of hip joint
centers.31 A minimum of 5 different static positions per leg were used to estimate hip
joint center with a maximum error rate of 0.01. If an error rate above 0.01 occurred, the
sensors were rechecked and digitization was repeated until it dropped below. The global
reference system was defined using the right hand rule for all body segments with the
positive x-axis defined as the posterior to anterior axis, the positive y-axis defined as the
inferior to superior longitudinal axis, and the positive z-axis as medial to lateral. The
Euler angle sequence was ZY’X”.
With the subjects standing with their arms relaxed at their sides, a minimum of
three landmarks per segment were palpated on the lower extremities and digitized with
the stylus. Bony landmarks on the femur, tibia and fibula were palpated and digitized for
transformation of sensor data to the local anatomic coordinate system.49,50 Sensor
landmarks are shown in Figures 1 and 2. Digitization of the bony landmarks allowed the
receiver data to be transformed from a global coordinate system to a local anatomic
coordinate system embedded in each of the bony segments. Local anatomic axes systems
are advantageous to compare segments to one another, allowing a clinically meaningful
50
comparison as opposed to the use of a global reference system. The local coordinate
system of the thigh was set up using the medial/lateral femoral epicondyles.49 The local
coordinate system for the shank was determined using medial and lateral malleoli and
medial/lateral joint lines.49 Data was captured at 100 Hz and low pass filtered at 30 Hz
using a Butterworth 4th order zero phase shift filter (optimal filtering for smoothing of
data without losing too many points, as noted with trials of different filter levels). Force
plate data was sampled at 1000 Hz, which has a default set up with an analog antialiasing filter of 500 Hz. The force plate was calibrated for each subject prior to
testing. No other software filtering occurred, as this was sufficient.
Figure 2. Sensor placement on the sacrum.
Figure 1. Sensor placement on the
distal, lateral thigh and mid shank.
Prior to testing, demographic information was collected from the subjects.
Maximal jump height was found by having the participant complete a bilateral (BIL)
standing vertical jump next to a wall. Jump height was measured against a wall with the
middle finger on the participant’s right hand being used to mark reach without jump and
51
peak jump height. Three jumps were completed and the average was taken. The
participants then completed a maximum of five bilateral drop jump vertical jumps which
was scored by the researchers using the Landing Error Scoring System- Real Time
(LESS-RT) protocol. The participants completed the jumps by bilaterally jumping
forward from a 30 cm box and landing on a marker which was 50% of the participant’s
height away from the box. Jumps were redone if the participant failed to land on the
marker or if the participant did not leave the box from both feet.
Task
Following the LESS-RT, the participants were marked for each sensor placement,
digitization occurred, and the subject was allowed 3-5 practice jumps for each condition
(BIL to BIL, BIL to single-leg right, and BIL to single-leg left). The subjects then
completed the jumping protocol. This protocol consisted of the subjects completing three
different jump maneuvers: BIL to single-leg (SL) on the right, BIL to SL on the left, and
BIL to BIL. Jump order was chosen at random prior to the testing session. Participants
positioned themselves with one foot on each force plate prior to each jump. Jumps were
considered a success if the participant was able to land with one or both feet (depending
on jump type) completely on the respective force plate (i.e. SL jump right would require
the right foot of the subject to land within the right force plate area). The participants
were instructed to jump as high as they can without looking down while trying to reach a
dowel positioned above their head. An attempt would be considered a failure if the
participant was unable to land on the respective force plate with their entire foot or if the
52
participant lost their balance within the first two seconds of landing. Four successful
jumps were completed in each direction. Following the initial data collection the fatigue
protocol was completed. After the participant completed the entire fatigue protocol, RPE
was taken to measure and ensure fatigue was reached, sensors were attached and digitized
for some subjects on the participant’s body, and the previous four jumps were completed
in each direction. The re-digitization occurred in cases where excessive sensor slippage
occurred and the computer animation displayed non-analyzable data.
Fatigue Protocol
The Functional Agility Short-Term Fatigue Protocol (FAST-FP) was utilized for
this study.37 The FAST-PF consisted of multiple agility exercises including: step-up onto
a 30 cm-height box, ‘L-drill’, vertical jumps, and agility ladder drills.37 One round
consisted of all four agility drills and four total rounds were completed. Participants
L-Drill
started by completing step-up movements
onto the 30 cm-height box for 20 seconds to
5 YARDS
5 YARDS
a metronome beat set at 200 bpm.
Immediately afterwards, the participants
completed one repetition of the ‘L-drill.’
The ‘L-drill’ involved three cones that were
placed in the shape of an L (as seen in
Start/Finish
Figure 3. Diagram of the fatigue
protocol L-Drill.
Figure 3). Each cone was 4.5 yards apart
from one another. To complete the drill, the
53
participant sprinted forward from the starting cone to the second cone. They immediately
turned around and sprinted back to the starting cone. Then, the subject turned again and
sprinted back to the second cone where they ran around it to cut to the left towards the
third cone. The participants ran in a circle around the third cone from the inside to the
outside, ran back to the second cone around the outside, and completed the sprint back to
the first cone.37 Immediately after the sprint, the participants completed five vertical
jumps at 80% of their maximal jump height. Height was controlled by having the
participants jump next to a wall and touch a marker on the wall at 80% of their maximal
jump height. After jumping, the subject immediately progressed to completing the ladder
agility drill. During the first and third rounds, the participants ran through the ladder
(down and back) forwards with both feet touching the inside spaces of the ladder. During
the second and fourth rounds the participant ran through the ladder sideways (facing
opposite directions each round), placing both feet in each ladder space. During all four
rounds a metronome was set to 200 bpm for the step ups and ladder drills. All four rounds
of the fatigue protocol were completed with no rest breaks in between them.37 RPE was
used to determine the level of fatigue each participant was feeling at the end of the
protocol (Mean = 8.4±0.75 using Modified Borg Scale). Immediately following the
protocol, RPE was taken and the second set of jumps were completed using the Motion
Monitor as described above.
54
Data Reduction
Raw kinematic data collected by the Motion Monitor system was selected for
export and further processing. Each trial was viewed and data was cut to capture only the
task requirements by a single investigator to ensure consistency. Original variables
included in the export file were pre and post fatigue protocol Euler angles (x, y, z),
normalized knee moments (x, y, z), and ground reaction force (y) for bilateral land, right
leg land and left leg land. Pre and post fatigue conditions were also coded into a separate
column for ANOVA analysis with all pertinent kinematic and kinetic data included per
condition by variable to use pre and post fatigue conditions as a factor.
Statistics
A paired t-test was used to analyze significant differences between all subjects for
pre and post fatigue conditions, as well as for each gender, in knee mechanics before and
after the fatigue protocol for bilateral, and right and left landing trials. The independent
variables were pre and post fatigue protocol, and landing style. The dependent variables
included knee angles and moments in all 3 planes, and ground reaction forces. An
independent t-test was also used to analyze significant differences between genders in
knee mechanics following the fatigue protocol for each landing style. The independent
variables were gender, while the dependent variables were knee angles, moments, and
ground reaction force differences between pre and post fatigue protocol. Significance was
set at a p-value of 0.05.
55
56
CHAPTER IV
RESULTS
Demographic data for participants is presented in Table 1.
Table 1. DEMOGRAPHICS OF PARTICIPANTS COMPARED BY GENDER
Variable
Male
Female
Age (years)
27.6 ±4.1
26.1 ±8.5
Height (inches)
69.9 ±4.0
64.5 ±21.3
Weight (pounds)
180.4 ±16.2
165.2 ±54.7
BMI (kg/m2)
26.2 ±3.4
23.1 ±7.3
Waist circumference (inches)
34.0 ±3.0
31.2 ±10.3
Vertical Jump (inches)
RPE for Fatigue Protocol (010)
LESS-RT Score (0-15)
21.2 ±4.0
19.0 ±6.5
8.4 ±1.0
7.7 ±2.5
7.6 ±3.2
7.8 ±3.2
Data represented as: means±SD
All Subjects Pre-Post Fatigue
A number of significant differences were found in knee kinematic and kinetic
values before and after the fatigue protocol for all subjects during the three types of
landings (Table 2). All significant results had a p-value equal to or less than 0.05; a
number had a p-value equal to or less than 0.01. Table 1 has the corresponding
statistics. Knee flexion and internal rotation angles significantly increased after the
fatigue protocol for left single leg landings, right single leg landings, and bilateral
landings only in the right leg. Knee external rotation and adduction angles significantly
increased for left single leg landings, right single leg landings, and bilateral landings in
57
both legs. Knee abduction angles significantly increased for right single leg landings,
and in both legs for bilateral landings. Flexion moments significantly increased after the
fatigue protocol only for left single leg landings. External rotation moments significantly
increased for both left and right single leg landings. Abduction moments significantly
increased for right single leg landings, and bilateral landings in the right leg. Extension
moments significantly decreased after the fatigue protocol only for right single leg
landings. No significant differences were found for internal rotation moments, adduction
moments, or ground reaction forces when comparing pre- and post-fatigue for all
subjects.
Table 2. KNEE KINEMATICS AND KINETICS FOR ALL SUBJECTS PREPOST FATIGUE
Bilateral Landing
Left
Right
Left Landing
Right Landing
Pre
Post
Pre
Post
Variable
Pre
Post
Pre
Post
Flexion
Angle
-
-
77.9±17.9
84.9±22**
60.8±11.3
64.7±17*
61±13.6
66.8±16.6**
IR Angle
-
-
8.9±4
12.5±12.2*
11±6.5
16±15.8*
7.8±3.4
12.3±10.7**
ER Angle
12.6±5.9
20.7±17**
13.3±4.9
20.5±17.8**
10±4.9
21.3±15.5**
12±4.8
20.8±18**
ABD Angle
11.8±10.5
16.9±14.3**
16.3±9.4
21±17.6**
-
-
15.1±8.3
17.8±11.1*
ADD Angle
4.8±3.5
10.8±10.5**
8.9±9
19.3±20.5**
4.7±3
12±10.3**
8.7±8.9
19.3±20.6**
-
-
-
-
0.69±0.26
0.79±0.36*
-
-
-
-
-
-
0.09±0.13
0.12±0.16*
11.4±15.5
15.8±22.9*
-
-
0.78±0.45
0.96±0.73**
-
-
0.63±0.61
0.85±0.72*
-
-
-
-
-
-
7.4±1.8
6.9±2.1*
Flexion
Moment
ER
Moment
ABD
Moment
Extension
Moment
Data represented as: means±SD, * = p<0.05, ** = p<0.01
IR = Internal Rotation, ER = External Rotation, ABD = Abduction, ADD = Adduction
Angles are measured in degrees, Moments are measured in Nm/kg
58
Males Pre-Post Fatigue
Significant differences were also found in kinematic and kinetic values pre- and
post-fatigue for male subjects during all three landings (Table 3). Once again, all
significant results had a p-value equal to or less than 0.05; a number had a p-value equal
to or less than 0.01. Table 3 displays the corresponding statistic for each value. External
rotation angles significantly increased post-fatigue for left single leg landings, right single
leg landings, and bilateral landings in the left leg only. Abduction angles significantly
increased in the left leg for bilateral landings. Adduction angles significantly increased
for left single leg landings, and bilateral landings in the left leg. Abduction moments
significantly increased for right single leg landings, and bilateral landings in the right leg
only. No significant differences were found for flexion and internal rotation angles,
flexion, internal rotation, external rotation, adduction and extension moments, or ground
reaction forces.
Table 3. KNEE KINEMATICS AND KINETICS FOR MALES PRE-POST
FATIGUE
Bilateral Landing
Left
Variable
Pre
Right
Right Landing
Pre
Post
Pre
Post
Post
Pre
Post
-
-
9±4.8
29.6±18.7**
10.5±5.1
23.9±21.3**
-
-
-
-
-
-
-
-
3.5±1.8
15.2±13.6**
0.62±0.25
0.9±0.8*
ER Angle
10.7±7
28.1±23**
ABD
Angle
ADD
Angle
ABD
Moment
18.3±11.2
25.1±17.3*
3.4±2.2
14.1±13.8**
-
Left Landing
-
-
Data represented as: means±SD, *=p<0.05, **=p<0.01
ER = External Rotation, ABD = Abduction, ADD = Adduction
Angles are measured in degrees, Moments are measured in Nm/kg
-
-
-
0.52±0.23
0.95±0.83**
59
Females Pre-Post Fatigue
Numerous significant differences were found in kinematic and kinetic values
before and after the fatigue protocol for female subjects during all three landings (Table
4). All significant results had a p-value equal to or less than 0.05; a number had a p-value
equal to or less than 0.01. The corresponding statistics are displayed in Table 4. Flexion,
external rotation and adduction angles significantly increased after the fatigue protocol
for all landings and legs. Internal rotation angles significantly increased for right single
leg landings, and bilateral landings in the right leg. Flexion moments significantly
increased for left single leg landings, and bilateral landings in the right leg. External
rotation moments significantly increased for left and right single leg landings. Internal
rotation moments significantly decreased post-fatigue for left single leg landings, and
bilateral landings in the left leg. Ground reaction forces within the knee were found to
significantly decrease for right single leg landings. No significantly differences were
found for abduction angles, or abduction, adduction and extension moments.
60
Table 4. KNEE KINEMATICS AND KINETICS FOR FEMALES PRE-POST
FATIGUE
Bilateral Landing
Left
Left Landing
Right Landing
Pre
Post
Pre
Post
Right
Variable
Pre
Post
Pre
Post
Flexion
Angle
80.8±15.9
86±17.6**
81.2±16.
8
89.2±16.8**
60.8±9.9
64.1±13*
58.9±10.4
66.8±16.5**
IR Angle
-
-
8±3.6
11.5±10.5*
-
-
6.8±3.3
13.6±11.3**
ER Angle
6.6±6
10.3±6**
13.5±8
17.5±7.9**
10.7±5
15.9±10.2
**
12.9±4.4
18.8±15.5*
5.7±3.9
8.8±7.2*
7.3±3.8
17±17**
5.4±3.4
9.8±6.8**
6.7±4.3
15.9±17.1**
-
-
0.68±0.1
6
0.77±0.23*
0.53±0.18
0.68±0.35
*
-
-
0.36±0.26
0.26±0.21**
-
-
0.5±0.45
0.4±0.32*
-
-
-
-
-
-
0.08±0.12
0.13±0.17
*
8.6±13.1
12.8±19.3*
-
-
-
-
-
-
3175.9±698
2989±541**
ADD
Angle
Flexion
Moment
IR
Moment
ER
Moment
GRF
Data represented as: means±SD, *=p<0.05, **=p<0.01
IR = Internal Rotation, ER = External Rotation, ADD = Adduction, GRF = Ground Reaction Forces
Angles are measured in degrees, Moments are measured in Nm/kg, GRFs are measured in N
Differences Between Gender Pre-Post Fatigue
Finally, significant knee kinematic and kinetic differences were found between
genders following the fatigue protocol (Table 5). Significant values were found for all
three landings. All significant results had a p-value equal to or less than 0.05; a number
had a p-value equal to or less than 0.01. Table 4 displays the corresponding
statistics. Males demonstrated significantly greater differences between pre- and postfatigue with flexion, external rotation and adduction angles when compared to
females. These greater differences occurred for bilateral landings in the left leg for
flexion angles, for left single leg landings for external rotation angles, and for both left
single leg landings and bilateral landings in the left leg for adduction angles. Males also
showed significantly greater differences with internal rotation and adduction moments for
61
bilateral landings in the left leg, and with abduction moments for right single leg
landings. For ground reaction forces, males had significantly greater values compared to
females for right single leg landings. No significant differences were found for the right
leg during bilateral landings. Also, there were no significantly differences for internal
rotation and abduction angles, or flexion, external rotation and extension moments.
Table 5. KNEE KINEMATIC AND KINETIC DIFFERENCES BETWEEN
GENDER POST FATIGUE
Bilateral Landing
Left
Variable
Flexion
Angle
ER Angle
ADD
Angle
IR
Moment
ABD
Moment
ADD
Moment
GRF
Left Landing
Right Landing
Male Female
Male Female
Right
Male Female Male Female
6.0
-5.2**
-
-
-
-
-
-
-
-
-
-
-13.8
-5.2*
-
-
-10.6
-2**
-
-
-10
-3**
-
-
-0.05
0.1**
-
-
-
-
-
-
-
-
-
-
-
-
-0.43
-0.03*
3.9
2.2**
-
-
-
-
-
-
-
-
-
-
-
-
-87.8
186.2*
Data represented as: means±SD, *=p<0.05, **=p<0.01
ER = External Rotation, IR = Internal Rotation, ABD = Abduction, ADD = Adduction, GRF = Ground
Reaction Forces
Angles are measured in degrees, Moments are measured in Nm/kg, GRFs are measured in N
62
CHAPTER V
DISCUSSION
Summary of Previous Research Findings
Extensive research has been conducted to determine if gender differences exist for
landing mechanics, if fatigue has a direct impact on injury risk through analysis of
VGRFs, hip/knee/ankle kinematic changes, jump height, etc., and a combination of both.
Biomechanical gender differences do exist. Females demonstrate stiffer landing styles
than their male counterparts. This single finding may have a profound impact on the ACL
injury rates in healthy males versus females. Stiffer landings typically equate to increased
ground reaction forces, which have been shown to increase rates of non-contact ACL
tears.28 Females also exhibit increased abductor moments and knee valgus, as well as
decreased hip and knee flexion upon landing.27,28, 30, 32,33,34
Fatigue, in studies, is mimicked in different ways. Some studies choose endurance
tasks such as running until failure is reached; others focus primarily on fatigue through
muscular activity such as squats, while some try a combination by simulating gameplay
as closely as possible by mixing endurance activities with plyometric movements to reach
a full body fatigue before measuring lower extremity (LE) movement patterns.19-22,30,36-39
Post-fatigue research with healthy subjects has observed decreased muscle activation of
the quadriceps and hamstrings, decreased jump height, and changes in knee kinematics.1922
Some studies demonstrate that females have higher risk movement patterns with
fatigue than males; however other studies show that both genders are equally impacted by
63
fatigue.39 One study compared dancers with other team sport athletes landing mechanics.
They showed that dancers took longer to reach a fatigued state, but no other group
interactions appeared statistically significant.39 A MANOVA showed female dancers
landed with a significantly lower knee valgus angle, hip adduction moment, and trunk
side flexion than female team sport athletes. However, there was no interaction with
fatigue and sex or group. Post hoc testing showed increased trunk flexion and lateral
trunk lean in a fatigued state of both the dancer group and athlete group. Fatigue, among
both groups, globally increased peak knee flexion angles and decreased knee flexion
moments.39 Another study reported that males had increased torque of the knee extensors
and higher MEP amplitudes of the hamstrings than females pre-exercise. Post-exercise,
males exhibited greater vastus lateralis activation and a decreased knee-extension torque
when compared to females. Authors suggested that there is a potential loss of quadriceps
torque after exercise, particularly in females, which suggests a difference in how each
gender reacts to fatigue. The researchers suggest that this could potentially create an
increased risk of knee injuries in females, similar to the previous study.36
James et al.21 found significantly greater knee flexion at contact after the
isometric fatigue protocol compared to their cycling protocol. The isometric protocol
was meant to induce short duration isolated fatigue, whereas the cycling protocol was
aimed to bring about whole-body fatigue that persisted into recovery. The knee flexion
finding suggests that the fatigue protocol has an impact on changes in kinematics and
highlights the importance that fatigue must be achieved in order to accurately study
changes in kinematics.21 McLean et al.22 found fatigue produced significant increases
64
peak knee abduction and peak knee internal rotation in both genders and legs. Fatigue
also produced significant increases in peak knee abduction and internal rotation moments
in both genders. The authors cited previous research that stated internal tibial rotation
movement contributes directly to ACL loading and, in turn, an increased risk of ACL
injury. Therefore, their results imply fatigue from their fatigue protocol increases ACL
loading and risk of ACL injury. They also found that females had significantly greater
increases in peak knee abduction and internal rotation, and had greater increases in peak
knee abduction with fatigue compared to males. The authors suggested that this
difference may contribute to females’ increased risk of ACL injury.22
When measuring patients post ACLR, fatigue has shown to alter knee
mechanics.23,24 Webster et al.23 found no significant differences between subjects with an
ACLR or the control subjects. Although, the ACLR group saw a decrease in peak hip
flexion and ankle dorsiflexion as well as an increase in hip abduction and knee abduction
and internal rotation (IR) on the operative side when compared to the control group, there
were no significant interactions between fatigue level and group/limb for any kinematic
variable. Fatigue did impact the kinetic component of both groups; decreases in knee
flexion and adduction moments were noted, showing that individuals are at a greater risk
of tearing both ACLs regardless or ACLR history. Smaller knee moments were noted in
the ACLR limb compared to the contralateral limb as well as an increased hip flexion
moment on the operated limb compared to a decrease in the control group. Webster et
al.23 hypothesized that this was evident of compensatory strategies occurring to help
preserve lower limb stability. Frank et al.24 demonstrated that individuals with ACLR are
65
at an increased risk of injury when exposed to fatiguing activity as evident by less hip
flexion at IC, which points to a reduction in muscular resistance to fatigue and the
neuromuscular system’s ability to sustain quality landing patterns after fatigue.
This research aimed to reconcile the movement pattern and fatigue protocol with
what is seen in sport to note differences between knee and hip kinematics and kinetics for
healthy men and women. The research compared the lower extremity kinematics and
kinetics of a jump-land task (double and single leg land) between healthy men and
women after a fatigue protocol. Ultimately, this research aimed to identify whether a
dysfunctional or high risk movement pattern existed which may predispose the subject
(male and/or female) to possible knee or ACL injury. Our hypothesis was that all subjects
will display changes in their movement patterns that would predispose them to knee
injury. However, we expected that women, as compared to men, would exhibit more
aberrant lower extremity kinematics and kinetics upon fatigue, further increasing their
relative risk of ACL injury. It is well supported that females demonstrate stiffer landings
characterized by increased ground reaction forces, decreased knee flexion angles, and
increased motions that are consistent with knee valgus.27,28, 30, 32,33,34 It was hypothesized
that these differences would be more pronounced in the female population.
Impact of Fatigue on Subjects
All subjects demonstrated increased knee flexion, internal rotation, external
rotation, adduction, and abduction angles post-fatigue protocol. Flexion, external rotation
and abduction moments also significantly increased post-fatigue. Extension moments
66
decreased after the fatigue protocol. There were no statistically significant changes in
ground reaction forces post-fatigue in this study. These findings are not necessarily
inconsistent with previous literature. Many studies support statistically significant
changes in ground reaction forces when fatigued, but the literature is inconsistent.20,21
Some studies support increased ground reaction forces when fatigued, while
others demonstrated decreased ground reaction forces post-fatigue.20,21 Other literature
supported no significant changes in ground reaction forces after a fatigue protocol, as
seen in the current study.19 This examination into the ground reaction force data between
fatigue literature indicates the inconsistent findings across this body of research.
Males demonstrated increased external rotation, abduction, and adduction angles
post-fatigue protocol. Additionally, abduction moments were increased. A study
completed by McLean et al.22 also found that males demonstrate increased abduction
angles and moments when fatigued. Females demonstrated increased flexion, external
rotation, adduction, and internal rotation angles. Flexion and extension moments
increased while internal rotation moments and ground reaction forces decreased
following the fatigue protocol. In opposition to this, McLean et al.22 found increased
internal rotation angles for female subjects with fatigue.
When comparing genders, the data supports that males demonstrate higher
adduction, external rotation, and flexion angles. Internal rotation, adduction, abduction
moments were also increased in the male population. Finally, males demonstrated greater
ground reaction forces when fatigued. These findings contradict the previous literature on
landing mechanics between males and females when the subjects are not fatigued.27,28, 30,
67
32,33,34
However, the post-fatigue landing literature is too varied at this point to accurately
portray the typical landing changes that occur when a subject is fatigued.22,36,39
These results do not closely align with the hypothesis. An explanation of these
findings is that men and women display decreased dynamic control for bilateral and
single leg landings when fatigued, resulting in greater angles and/or moments overall. A
relatively new body of research is examining how much variability is required to safely
land on different surfaces without increasing injury risk. Movement variability was
previously thought of as ‘noise’ that should decrease as an athlete becomes more
proficient at a given task.51-54 However, recent literature has supported that high level
athletes have a certain amount of compensatory variability, while beginners have a more
consistent pattern that is typically more rigid.54 Given the results, it may suggest that
males demonstrate increased variability that serves them well in sport, consistent with
compensatory variability. Females may have a more consistent, yet rigid, pattern that
may increase knee injury risk. Previous literature has supported this hypothesis, as
females have demonstrated less hip and knee flexion with dynamic valgus at the knee
during landing tasks when compared to males.27,28,30,32-34
Gender differences noted in our specific research population may have impacted
the results. The first noted observation was a different level of athleticism and body
awareness between the male and female subjects. The second observation arose from the
data collection process. Participants were instructed to jump as high as they could and
land in the center of the force plate. It was noted that males typically jumped higher and
68
exerted greater effort than females, while the female participants typically required less
trials to land in the center of the force plate.
Limitations
This study revealed several limitations. One of which includes a small sample size
with a narrow age range. These two factors decrease the power of the results and reduce
the ability to generalize our findings to other populations. Another limitation is the
possible interference of skin motion artifact as bone pins were not utilized to more
accurately measure joint angles. Given human involvement, there is the potential for error
in regards to placement of sensors and validity of trials. Additionally, only some subjects
were re-digitized after the fatigue protocol. Due to the need for re-digitization and
multiple trials post-fatigue, there was increased recovery time, which may decrease the
validity of the post-fatigue data. The nature of the study required the subject to land on
force-plates, which increased the complexity of the task. This ultimately increased the
number of trials and a provided a potential for change in jump and landing mechanics in
order to land in the center of the force plate. Finally, hip and ankle mechanics were not
analyzed in this study. Those joints may provide additional insight to the landing
mechanisms that differ between men and women when fatigued. Further research is
recommended, specifically with an increased sample size to increase the power of our
results and to further evaluate the concepts of movement variability suggested in this
study.
69
CHAPTER VI
CONCLUSION
This study supports that both gender and fatigue impact landing mechanics at the
knee, which are consistent findings in previous literature. Furthermore, this study
suggests that movement variability may influence landing patterns in men more than
women. Further research is warranted to explore the relationship between fatigue and
movement variability between genders when landing.
Our hypothesis was that all subjects would display negative changes in their
movement patterns. However, we expected that women, as compared to men, would
exhibit more aberrant lower extremity kinematics and kinetics upon fatigue, further
increasing their relative risk of ACL injury.
The collected data did not necessarily support our hypothesis. Men exhibited
more varied landing mechanics, while women’s landing patterns tended to be more
consistent and rigid. These findings pertain to a larger discussion surrounding movement
variability. It has yet to be determined if and how much movement pattern variability
post fatigue play in the reduction of risk for lower extremity injuries.
Future research should build on this study with a larger subject base. The
relationship between landing mechanics and fatigue has yet to be determined. It is
important to continue this body of research to understand the mechanics leading to
increased injury ACL injury rates among female athletes.
70
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