THE EFFECT OF FATIGUE ON LOWER EXTREMITY MECHANICS DURING THE
UNANTICIPATED CUTTING MANEUVER
A THESIS SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL
FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE
MASTER OF SCIENCE
BY
KAITLYN J WEISS
D.CLARK DICKIN
BALL STATE UNIVERSITY
MUNCIE, INDIANA
MAY 2013
THE EFFECT OF FATIGUE ON LOWER EXTREMITY MECHANICS DURING THE
UNANTICIPATED SIDECUTTING MANEUVER
A THESIS SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE
MASTER OF EXERCISE SCIENCE
BY
KAITLYN J. WEISS
COMMITTEE APPROVAL:
Committee Chairperson (Dr. D. Clark Dickin)
Date
Committee Member (Dr. Henry Wang)
Date
Committee Member (Dr. Dorice Hankemeier)
Date
DEPARTMENTAL APPROVAL:
Departmental Chair Person
Date
GRADUATE OFFICE CHECK:
Dean of Graduate School
Date
BALL STATE UNIVERSITY
MUNCIE, INDIANA
May 2013
Table of Contents
CHAPTER 1: INTRODUCTION .............................................................................................................. 3
PURPOSE ..................................................................................................................................................... 3
SIGNIFICANCE ............................................................................................................................................. 3
HYPOTHESIS ............................................................................................................................................... 3
LIMITATIONS .............................................................................................................................................. 3
DELIMITATIONS .......................................................................................................................................... 3
ASSUMPTIONS ............................................................................................................................................. 3
OPERATIONAL DEFINITIONS ....................................................................................................................... 3
CHAPTER 2 : REVIEW OF THE LITERATURE ................................................................................. 3
INTRODUCTION ........................................................................................................................................... 3
CUTTING MANEUVER ................................................................................................................................. 3
Deceleration ........................................................................................................................................... 3
Plant and Cut ......................................................................................................................................... 3
Takeoff .................................................................................................................................................. 3
ANTICIPATED VS. UNANTICIPATED TASKS.................................................................................................. 3
FATIGUE ..................................................................................................................................................... 3
Central fatigue ....................................................................................................................................... 3
Peripheral fatigue ................................................................................................................................... 3
Fatigue and sports .................................................................................................................................. 3
GENERAL ANATOMY OF THE KNEE............................................................................................................. 3
GENERAL ANKLE ANATOMY ...................................................................................................................... 3
GENERAL HIP ANATOMY ............................................................................................................................ 3
GENDER ...................................................................................................................................................... 3
Anatomical Differences ......................................................................................................................... 3
Hormonal Differences ........................................................................................................................... 3
Neuromuscular Differences ................................................................................................................... 3
GENERAL MECHANISMS OF ACL INJURY ................................................................................................... 3
CONCLUSIONS............................................................................................................................................. 3
CHAPTER 3: MANUSCRIPT ................................................................................................................... 3
ABSTRACT .................................................................................................................................................. 3
INTRODUCTION ........................................................................................................................................... 3
METHODS ................................................................................................................................................... 3
RESULTS ..................................................................................................................................................... 3
DISCUSSION ................................................................................................................................................ 3
CONCLUSION .............................................................................................................................................. 3
PRACTICAL IMPLICATIONS .......................................................................................................................... 3
Acknowledgments ................................................................................................................................. 3
TABLES AND FIGURES ................................................................................................................................. 3
CHAPTER 4: SUMMARY AND CONCLUSIONS ................................................................................. 3
3
CHAPTER 5: REFERENCES ................................................................................................................... 4
4
Chapter 1: Introduction
The cutting maneuver is a dynamic sports task that allows the athlete to change
direction from standing, walking, or running. Running combined with the cutting
maneuver increases the strain and places the ligaments of the knee at an elevated risk of
injury.1 The sidestep cut is performed by planting with the foot opposite the intended
change in direction while the crossover cut is accomplished by planting with the foot on
the same side as the intended change in direction.1 The sidestep cut is the more common
of the two cuts and is composed of three separate phases: deceleration, plant and cut, and
takeoff.1
The deceleration phase of the cut may occur over a few to several running gait
cycles beginning with the foot to be planted for the cut. The primary goal during
deceleration is to decrease the athlete’s momentum by using the greatest amount of force
possible in the shortest amount of time so they can begin to move in the new direction 2.
This takes place over a few shortened gait cycles, leading to larger braking forces and
increased ground contact time to attenuate the large eccentric forces while placing as
little stress on the joints in the lower extremity as possible 2. During the deceleration
phase anterorposterior forces are placed on the knee joint causing the posterior cruciate
ligament (PCL), capsular ligaments, and the collateral ligaments to tighten in response to
anterior translation of the femur relative to the tibia 1. Through an in-vivo anterior
cruciate ligament (ACL) strain case study it was found that strain on the ACL increased
during the flight phase while the knee was extended and pre-activation of the quadriceps
and gastrocnemius occurred. Though the strain that occurred during the flight phase was
not significant, the strain continued to increase during the landing phase prior to reaching
is peak at the same time as the peak ground reaction force, and remained high during the
stance phase 3.
The athlete achieves a change in the direction of their momentum during the plant
and cut phase with the plant leg providing the chief acceleration force in that direction.
During this phase, a large amount of stress is applied medially to the capsular ligaments
of the knee 1. Several studies have indicated an elevated risk for knee injury during the
cutting maneuver when flexion, valgus/varus, and internal and external rotation loads act
on the knee 4-6. The researchers ascertained that external flexion loads when
accompanied by greater valgus and internal rotation moments have the strongest
influence on the magnitude of stress applied to the ACL. This occurs mainly when the
weight is transferred to the stance leg and the knee is near full extension 7, 8. In a study
looking at the correlation between lower extremity posture during initial contact and peak
knee valgus moment, the researchers noted that larger peak valgus moments were found
in conjunction with larger initial contact hip flexion, internal rotation, and knee valgus,
potentially indicating a higher risk of injury 5. It has also been observed, both in-vivo and
in-vitro, that applied valgus or varus moments accompanied by weight bearing, anterior
shear force, or quadriceps contraction increases ACL strain. Increased ACL loading has
also been identified when knee valgus or varus loading is combined with anterior sheer
force of the tibia. The examined effect of combined frontal plane and transverse plane
loading has also been considered as these motions have been seen together at the time of
ACL injury 9-11. Researchers using a robotic arm to look at tensile forces acting on the
6
ACL found increased forces when valgus loading occurred in tandem with either knee
internal rotation loads or external rotation loads 12.
Several authors have sought to refine the ecological validity of their work by
using unanticipated tasks to minimize the learning effect, and gauge the kinematic and
kinetic alterations made by the subjects when presented with a real-time situation 13, 14.
The use of unanticipated dynamic tasks such as the cutting maneuver is meant to mimic
the nature of the tasks’ performances in game situations 15, 16. Previous research in the
area of motor control and the response by the central nervous system (CNS) to
anticipated and unanticipated actions indicate that a feed-forward mechanism is used in
reaction to anticipated actions or stimuli and that the CNS utilizes a preprogrammed plan
for anticipated postural modifications 8, 17, 18. Essentially, anticipation of a movement can
lead to altered reflex responses and postural modifications in order to decrease the
impending perturbation and uphold the necessary posture 17. A previous study
discovered that temporal constraints (self-initiated, anticipation-coincidence, and reaction
condition) have a significant effect on anticipatory postural adjustments, which are
centrally produced as a feed-forward mechanism to offset the mechanical effects of
predicted perturbations on stability in dynamic sports tasks. It was noted that the
unanticipated reaction condition led to the longest time to complete the anticipatory
postural adjustments when compared to the other two conditions 19. This may potentially
correlate to a longer duration of time between unanticipated tasks and postural
adjustments to offset the impending stresses incurred during the action.
Joint coordination is another aspect of motor control involved in dynamic
movements that is impacted by anticipation. Research observing joint coordination has
7
indicated that coordination variability provides flexibility in adjusting to perturbations
and is central to the alterations in coordination patterns that occur during movement 20, 21.
Thus, if an individual displays decreased coordination pattern variability during an
unanticipated task, it is possible that their ability to adjust to the environmental
perturbations commonly experienced during a soccer game or practice may be restricted
making them more susceptible to potential injury.
In a previous study looking at the anticipatory effects of running and cutting
maneuvers on joint loading, the authors found that knee joint moments increased during
unanticipated conditions when juxtaposed to preplanned maneuvers. The researchers
established that unanticipated maneuvers modified the external moments acting on the
knee as a result of the decreased time allotted to generate the proper postural adjustment
tactics. It was as a result of these insufficient postural alterations that potentially led to
decreased performance of the unanticipated cutting tasks (i.e., decreased cutting angle,
decreased speed) and enhanced external loads applied to the knee joint (i.e., valgus,
varus, internal rotation, and external rotation loads) in comparison to preplanned tasks 7.
As an individual performs a motor task(s) they will often times experience some
level of fatigue depending on the nature and duration of the task. During higher-intensity,
short duration activities, fatigue will primarily occur peripherally. Peripheral fatigue is
correlated to the capability of a muscle to perform physical work. As this fatigue occurs,
there is impairment to the normal functionality of the nerves and the muscles that are
contracting, which translates to a decline in the muscle’s ability to generate force due to
the incapability of the body to meet the increased energy demand in the contracting
muscles. The most often suggested mechanisms of peripheral fatigue consist of muscle
8
fiber type and its distribution, the readily available energy supply, the length of the
muscle, and the muscle’s strength prior to fatigue 22-24. If the task continues at a high
level for an extended period the individual may experience central fatigue. At the level
of the spinal cord central fatigue results in impaired alpha motor neuron firing or
suboptimal rate of recruitment to generate appropriate muscle force 25-27. Central fatigue
from impaired alpha motor neuron firing can lead to either loss of recruitment or
synergistic activation of multiple muscles 27. The implications for repetitive tasks such as
gait are that the overall rate slows and the time it takes to complete the task (e.g., the gait
cycle) increases with fatigue 28.
While both central and peripheral fatigue take place during sustained maximal
exertion muscle activity, during sub-maximal exertion peripheral fatigue occurs more
often than central fatigue 29-31. In the case of a soccer game, it is presumable that while
both central and peripheral fatigue will occur for an individual during a game, peripheral
fatigue will be a greater factor due to the inherent quick bursts of activity followed by
periods of recovery. By utilizing a fatigue protocol that primarily elicits peripheral
fatigue, the observations may be more applicable to true sport conditions. Fatigue has
also been regarded as a primary component affecting the musculoskeletal system as a
result of its connection to diminished proprioception and greater joint laxity 32 and the
subsequent reduction in the skeletal muscle’s ability to generate force 28, 33.
Previous studies looking at the effect of fatigue on various sports tasks have
utilized different protocols to simulate game fatigue. This has led to inconsistent findings
regarding joint dynamics during these tasks. Some studies have noted a significant
decrease in knee flexion after fatigue 34, where others have not observed fatigue effecting
9
maximum knee flexion 6. The variation in fatigue protocols likely contributes to the
incongruous mechanical observations of the knee joint during the sidecutting maneuver
35
. The two main types of fatigue protocols used to study the effect of fatigue on
neuromuscular control strategies are short-term and long-term 36, 37. Short-term protocols
used to stimulate fatigue may include a series of explosive tasks including vertical jumps
and sprints, squats and jumps, and single leg squats, whereas some long-term protocols
have utilized 60 minute shuttle runs and prolonged intermittent shuttle runs involving
periods of walking, jogging, running, and sprinting repeatedly for 60 to 90 minutes 38, 39.
In a previous study, both short-term and long-term fatigue protocols elicited altered hip
and knee kinetic and kinematics during sidestep cutting in female soccer players, and,
after only five minutes of performing either protocol, mechanical changes in the lower
extremity took place that could elevate the athletes risk for injury 40.
Previous research has also assessed lower-limb dominance related to the risk of
ACL injury and athletic performance41-43. Though there have not been significant
findings with respect to limb-dominance and injury risk, several of the authors have
stressed that limb asymmetries should be monitored and addressed as these differences
can pre-dispose athletes to athletic injury 41, 42, 44. Research by Ross et al., comparing
biomechanical variables between dominant and non-dominant limbs found that the
dominant limb had greater thigh strength, greater knee flexion range of motion, and
enhanced proprioception. Thus, by identifying kinetic and kinematic differences, preand post-fatigue between limbs may enhance our knowledge and abilities to design
preventative techniques for training, rehabilitation, and conditioning 45.
10
An additional factor related to knee, and more specifically ACL injuries is that of
gender. In comparison to males, female athletes have been shown to be four to six times
more prone to injure their ACL 46. When performing the sidestep cutting maneuver,
women have been found to have decreased hip flexion and abduction, and larger internal
rotation when compared to men, predisposing them to a potentially higher risk of injury
47
. Additionally, women have been found to have less coordination variability during an
unanticipated sidecutting task 48. A reduction in the variability of intra-limb couplings
suggests a more rigid coordination pattern, which could lead to an increased risk of lower
extremity injury 20. The authors determined that the gender differences in lower
extremity coordination variability, predominantly observed in the initial loading phase of
an unanticipated cutting maneuver, might attribute to the greater incidence of noncontact
ACL injuries in women. Further, it was suggested that these inflexible coordination
patterns in women during a competitive game may hinder their ability to properly adjust
to the frequent external perturbations common during sports play resulting in injury 48.
Based on these previous findings, it is possible that dampened lower extremity
coordination variability when coupled with fatigue during a game, could potentially lead
to an even greater likelihood of injury.
Purpose
The purpose of this study was to investigate collegiate level female soccer and
field hockey players and the effect of fatigue on lower extremity mechanics during
unanticipated sidecutting maneuver. Female athletes have a much greater incidence of
injury, particularly non-contact ACL injury, in comparison to males. Since the greatest
risk of injury is during the final phase of a game or practice, it is important to observe the
11
mechanical changes that occur as a result of fatigue. Since the majority of the dynamic
tasks occurring in a game are not pre-planned, this study sought to look at unanticipated
tasks to simulate practice and game conditions. The secondary purpose of this research
was to look at the impact limb dominance under these conditions.
Significance
Though there are many studies that look at the effect of fatigue on lower
extremity mechanics during dynamic sports tasks, there is a lack of research regarding the
effect of unanticipated actions coupled with fatigue. Further, there is a lack of research
looking at differences between the non-dominant and dominant limbs across all three
lower extremity joints under such conditions. Due to the nature of sports, it is more
realistic to consider that most athletes will not be able to pre-plan these dynamic tasks
during a game. Thus, it is important to increase the ecological validity and practical
implications of the research in this way. Since it is during these unanticipated tasks
towards the end of a game that most non-contact injuries occur, it is important to
investigate how fatigue impacts mechanics in these instances. Previous studies utilizing
anticipated tasks did not observe significant differences in movement mechanics between
pre and post fatigue conditions. More recent research looking at unanticipated tasks and
mechanical differences impacted by fatigue have only evaluated one or two of the joints
in the lower extremity, and only on the dominant limb. Furthermore, there is a lack of
research looking at the kinetic and kinematic changes that occur across the ankle, knee,
and hip during unanticipated and/or fatigue conditions with respect to limb dominance.
By comparing the lower extremity kinetics of a sidecutting maneuver both pre- and postfatigue and between the dominant and non-dominant limbs, it may be possible to isolate
12
specific changes that may place individuals at a greater risk of injury. The purpose of
this study was to investigate the effect of fatigue during the unanticipated sidecutting
maneuver on lower extremity mechanics. Findings of this study may provide valuable
insight for coaches and clinicians working with athletes at high risk of ACL injury.
Hypothesis
We hypothesized that fatigue would alter hip, knee, and ankle joint kinetics and
kinematics such that: the knee would experience greater external loads (e.g.,valgus,
varus, internal rotation, and external rotation loads), greater valgus knee torque and more
shallow knee flexion angles; the hip would have decreased flexion and abduction, and
larger internal rotation; and the ankle would exhibit greater inversion.
Limitations
The limitations of this study included: the sample population; the sample was
non-random and very specific which can affect its external validity; and effort by the
participants during the fatigue protocol could not be fully controlled.
Delimitations
The delimitations of this study included using only college aged, female field
hockey and soccer players; the study was conducted in a laboratory setting; and the
fatigue protocol used was the Yo-Yo Intermittent recovery test to simulate a soccer
and/or field hockey match.
Assumptions
The researchers assumed that the participants honestly answered the preparticipation health assessment questionnaire; participants used full effort during the
13
fatigue protocol; and the reliability and validity of the instruments and that they were
used properly.
Operational Definitions
. Dominant Limb-the patient’s preferred kicking leg 43
. Non-Dominant Limb-the stance leg opposite the patient’s preferred kicking leg
14
Chapter 2: Review of the Literature
Introduction
The purpose of this research was to look at female soccer and field hockey players
and the effect of fatigue on lower extremity mechanics during unanticipated sidecutting
maneuver. Female athletes have a much greater incidence of injury, particularly noncontact ACL injury, in comparison to males. Since the greatest risk of injury is during
the final phase of a game or practice, it is important to observe the mechanical changes
that occurs pre and post-fatigue. Since the majority of these dynamic tasks are not preplanned, this study seeks to look at unanticipated tasks to simulate practice and game
conditions.
Cutting Maneuver
The cutting maneuver is a dynamic sports task that allows the athlete to change
direction from standing, walking, or running. Running combined with the cutting
maneuver is the most dangerous of the three as it places the ligaments of the knee at the
greatest risk of injury 1. Depending on the sport, the maneuver varies in terms of the
degrees of the cut and its actual performance. The two primary cutting techniques are the
sidestep cut and the crossover cut. The sidestep cut is performed by planting with the
foot opposite to the intended change in direction while the crossover cut is accomplished
by planting with the foot on the same side as the intended change in direction 1. Though
the outcome is the same for both techniques, their execution during the plant and cut
phase and the imposed stresses vary. Both techniques are composed of three separate
phases: deceleration, plant and cut, and takeoff 1.
Deceleration
The deceleration phase may occur over several running gait cycles beginning with
the foot to be planted for the cut. The primary goal during deceleration is to decrease the
athlete’s momentum by using the greatest amount of force possible in the shortest amount
of time so they can begin to move in the new direction 2. Ultimately, rapid deceleration
of this nature takes place over a few shortened gait cycles, leading to larger braking
forces and increased ground contact to attenuate the large eccentric forces with little
stress to the joints in the lower extremity as possible 2. As the athlete enters the second
phase, known as the recovery phase in running gait, the descending foot experiences no
backward motion prior to foot strike and the knee is extended. At foot strike the foot
quickly plantar flexes so full contact is made with the ground and the lower extremity is
ahead of the athlete’s center of mass, opposing the body’s forward momentum 2. Upon
ground impact, the hip and knee flex and ankle dorsiflexion occurs so as to disperse the
impact forces over as many joints as possible which decreases the size of the stress and
allows the muscles to do increased negative work 2. Since force can only be produced by
the body while the foot is in contact with the support surface, the swing leg experiences
less time in noncontact to facilitate the need for greater contact or stance phase 2. As the
body is traveling forward, a deceleration force is created and the torso becomes more
upright and the foot dorsiflexes so the tibia angles anterior to the vertical axis and the
deceleration force reaches its maximum. This deceleration power is generated primarily
by the quadriceps and gastrocnemius 49. The body’s pre-activation of these muscles prior
to ground contact add to the body’s ability to absorb the extensive eccentric forces during
16
ground contact. During this portion of the cycle, the body’s kinetic energy decreases as
the negative velocity reaches zero prior to the propulsive phase. The body’s kinetic
energy is shifted to elastic energy which is then used for the following movement (e.g.,
the change in direction) 2. Decreased ankle dorsiflexion requires increased knee flexion
in order to keep the body’s center of mass behind the stance foot. The anterior position
of the foot relative to the center of mass leads to greater horizontal braking ground
reaction force. This force is dissipated by ankle dorsiflexion and knee and hip flexion
which alleviate the amount of stress applied to the body. The result is a slowing of the
forward momentum of the body which will ultimately lead to complete termination of
momentum in that direction 2. As the center of mass travels ahead of the foot, the planted
foot acts only to support the body’s weight. The second step in the deceleration phase
has no change in velocity and is considered passive 1.
During the cutting maneuver, deceleration creates anterorposterior forces on the
knee joint. These forces are generated while the foot is planted and the ankle is
dorsiflexed just beyond its neutral position. As deceleration is occurring, force is
posteriorly applied to the femur so as to inhibit anterior subluxation at the knee joint.
The force originates from the extensor mechanism which creates force through the
patellofemoral joint with some assistance from the gastrocnemius and the deceleration
force is applied through the attached ligaments. The posterior cruciate ligament (PCL),
capsular ligaments, and the collateral ligaments tighten in response to anterior translation
of the femur relative to the tibia 1. An in-vivo ACL strain case study found that strain on
the ACL increased during the flight phase while the knee is extended and pre-activation
of the quadriceps and gastrocnemius occurs. Though the strain that occurred during the
17
flight phase was not significant, the strain continued to increase during the landing phase
prior to reaching its peak at the same time as the peak ground reaction force took place,
and continued to remain fairly high during the stance phase 3.
Plant and Cut
The athlete achieves change in the direction of their momentum during the plant
and cut phase. The plant foot controls the ultimate deceleration in the initial direction
while the hips cause the torso to rotate in the direction the athlete plans to travel. The
athlete then swings the free leg in that direction which generates some acceleration. The
plant leg then provides the chief acceleration force in that direction 1.
During this phase, high levels of stress are applied to the capsular ligaments. Due
to mechanical differences, stress is medially placed during the sidestep cut, and laterally
placed during the crossover cut 1. In order to understand the effect of these forces, it is
best to create a distinction between the two cutting techniques.
sidestep cut.
In the sidestep cut, the player begins with the body in an upright posture, the hips
in flexion, and the knee of the plant leg, or the stance leg, in full extension. The first step
during this phase replicates that of the deceleration phase. The athlete maintains their
center of mass posterior to the plant foot, which terminates their travel along the initial
path. As this deceleration occurs, the torso and pelvis are internally rotated with respect
to the femur causing the body’s direction to change and allowing acceleration of the free
leg in that new direction. As this occurs, the femur is flexed and externally rotated, the
knee is flexed approximately 60 degrees, and the ankle is dorsiflexed. The athlete then
pushes off with the stance leg causing its acceleration in the new direction while the knee
18
and hip extend and the ankle reaches full plantar flexion 1. Once the hips and torso have
completely rotated in the new direction, the free leg swings in that same direction which
causes acceleration along that path as well as rotational deceleration 1.
When the pelvis is internally rotated, it creates an external rotational torque on the
femur that augments the deceleration forces on the medial side of the knee and decreases
forces on the lateral side of the knee. These additional stresses act on the medial
ligaments of the stance leg. For an athlete with anteromedial rotational instability, this
creates a greater risk of injury when performing this technique with the unstable knee for
the stance leg 1.
kinetics & kinematics.
Research conducted by Besier, Lloyd, Cochane, and Ackland indicated an elevated risk
for knee injury during the cutting maneuver when flexion, valgus/varus, and internal and
external rotation loads acted on the knee. The researchers ascertained that external
flexion loads when accompanied by greater valgus and internal rotation moments have
the strongest influence on the magnitude of stress applied to the ACL, namely when the
weight is transferred to the stance leg and the knee is near full extension 7, 8. In a study
looking at the correlation between lower extremity posture during initial contact and peak
knee valgus moment, the researchers noted that there were larger peak valgus moments in
conjunction with larger initial contact hip flexion, internal rotation, and knee valgus
angles 5.
crossover cut.
In the crossover cut, the pelvis is rotated externally relative to the stance leg. This is
accomplished by rotating the pelvis and torso via the internal femoral rotators. Unlike
19
sidestep cutting, pelvic rotation can also be performed by adducting and internally
rotating the free leg. The first part of this phase is also identical to the deceleration phase
as the stance leg decelerates the athlete with the ankle dorsiflexed, the hip and knee in
flexion, and the tibia angled anterior to the vertical axis. As this deceleration occurs, the
pelvis is externally rotated relative to the femur of the stance leg with the hip and knee in
flexion and the ankle in dorsiflexion. Once full rotation has been achieved, the free leg
begins its forward swing following normal running gait, which accelerates the body in the
new direction. Following this, the athlete then accelerates along the new path by pushing
off with the stance leg via full extension of the hip and knee and plantar flexion of the
ankle. This positions the athlete’s center of mass anterior to their center of pressure 1.
The stresses imposed on the knee during the crossover cut are like those generated
in the sidestep cut, but on the lateral side of the knee. The lateral ligaments take on the
additional forces while forces decrease on the medial side of the knee. Likewise, an
athlete with anterolateral rotational instability would experience difficulty performing
this technique using the unstable knee for the stance leg 1.
Takeoff
The takeoff phase is quite similar to normal running gait. It includes a support
phase and a running phase. The support phase is divided into three events including foot
strike, midsupport, and takeoff while the recovery phase includes follow through, forward
swing, and foot descent. The key difference between this phase and normal running gait
is that the athlete must have a greater forward lean so as to enhance acceleration in the
new path of travel 1.
20
Anticipated vs. Unanticipated Tasks
More recently, researchers have begun to consider the generalization of their
findings based on the ecological validity of their study designs 14, 50. Several authors
have sought to refine the ecological validity of their work by using unanticipated tasks to
minimize the learning effect, and gauge the kinematic and kinetic alterations made by the
subjects when presented with a real time situation 13, 14. The use of unanticipated
dynamic tasks such as the cutting maneuver is meant to mimic the nature of the tasks
performance in game situations 15, 16.
Previous research in the area of motor control and the response by the central
nervous system (CNS) to anticipated and unanticipated actions indicate that a feedforward mechanism is used in reaction to anticipated actions or stimuli and that the CNS
utilizes a preprogrammed plan for anticipated postural modifications
8, 15, 18
. A feed-
forward mechanism can be regarded as learned anticipatory responses to known cues.
Essentially, anticipation of a movement can lead to altered reflex responses and postural
modifications in order to decrease the impending perturbation and uphold the necessary
posture 17. In a study by Ilmane and LaRue, the researchers discovered that temporal
constraints (self-initiated, anticipation-coincidence, and reaction condition) have a
significant effect on anticipatory postural adjustments which are centrally produced as a
feed-forward mechanism to offset the mechanical effects of predicted perturbations on
stability in dynamic sports tasks. It was noted that the reaction condition required the
longest time for the individual to make the anticipatory postural adjustments when
compared to the other two conditions 19. Research observing joint coordination has
indicated that coordination variability provides flexibility in adjusting to perturbations
21
and is central to the alterations in coordination patterns that occur during movement 20, 21.
Decreased coordination variability has been present in individuals with knee pain as
opposed to those without 20. Thus, if an individual displays decreased coordination
pattern variability during an unanticipated task, it is possible that their ability to adjust to
the environmental perturbations commonly experienced during a soccer game or practice
may be restricted making them more susceptible to potential injury.
In a study by Besier, Lloyd, Ackland, & Cochrane looking at the anticipatory
effects on knee joint loading during running and cutting maneuvers, the authors found
that knee joint moments increased during unanticipated conditions when juxtaposed to
preplanned maneuvers 7. The researchers established that unanticipated maneuvers
modified the external moments acting on the knee as a result of the decreased time
allotted to generate the proper postural adjustment tactics. It was as a result of these
insufficient postural alterations that potentially led to the decreased performance
measures of the unanticipated cutting tasks (i.e., decreased cutting angle, decreased
speed) and enhanced external loads applied to the knee joint (i.e., valgus, varus, internal
rotation, and external rotation loads) in comparison to preplanned tasks. In an effort to
more aptly imitate an ecological environment, Besier et al., employed an unanticipated
cutting maneuver which resulted in knee moments up to twice the magnitude as
compared to those under preplanned conditions like those done by Maliznzak et al., and
McLean et al. 7, 51, 52 However, this study was limited to a male population 8. Research
looking at the differences between unanticipated and anticipated lower extremity
biomechanics during a sidestep cutting task determined that the unanticipated condition
displayed larger knee abduction angles, knee internal rotation, and hip abduction and
22
decreased knee flexion angles 16. The investigators theorized that there may be a greater
demand placed on the neuromechanical system when decision making is required.
Fatigue
In general, fatigue is considered to be the inability for an individual to maintain a
desired level of intensity during a task. Muscle fatigue is described as the loss of
maximum force-generating capacity, which may occur for a variety of reasons. Fatigue
is variable between individuals as well as the muscle groups and their subsequent fiber
types. Depending on the task, muscular fatigue is attributed to central (the brain, brain
stem and spinal cord) and/or peripheral (the actual muscles) mechanisms 28. It is known
that a large portion of this muscular fatigue results from processes occurring internally in
the muscle such as interruptions in the excitation-contraction coupling, accumulation of
metabolites, and depletion of muscle glycogen 53. When the task results in an
individual’s inability to completely activate a muscle voluntarily, the mechanism is
considered to be central fatigue, which involves events occurring in the brain and spinal
cord 54. Peripheral fatigue refers to a reduction in a muscle’s force production, due to
events occurring within the motor unit 54. Both central and peripheral fatigue is
generated during intermittent maximal muscle actions 29. During intermittent
submaximal muscle actions with an ample recovery time between each action, the
resulting fatigue has been identified as resulting from peripheral mechanisms 31. The
effect of fatigue on reflexes and coordination impairs an individual’s performance. If the
reflex arc is repeatedly stimulated, it reaches a point where it fails to elicit any type of
expected reflex response. It has been noted that the greater the number of interneurons
and synapses involved, the more quickly this reflex arc is fatigued. Coordination is
23
effected in the same way and irradiation of motor impulses to neighboring motor nerve
centers leads to a loss in coordination. The relationship between the intensity of the work
and endurance may be a fundamental component of performance31.
Both central and peripheral fatigue inhibits an individual’s performance. Central
fatigue, or neuromuscular fatigue, can cause an individual to feel tired or exhausted and
lead to their capacity to perform an activity or task to be reduced. Peripheral fatigue
inhibits their performance by reducing an individual’s ability to produce force despite
adequate motivation to execute the desired task.
Central fatigue
Central fatigue occurs during lower-intensity, longer duration activities. Several
mechanisms have been proposed to cause central fatigue. Suboptimal facilitation from
the motor cortex, desensitization of the motoneurons, greater inhibition from group III
and IV afferents (which measure the velocity of stretch of a muscle fiber), and reduced
facilitation from muscle spindles have all been suggested 29, 55-57. Decreased excitation of
the Ia afferent neurons resulting from a reduction in the firing frequency of muscle
spindles is one such mechanism 57. Another possible consideration is a decrease in the
size of the excitatory postsynaptic potential generated by each Ia afferent action potential
during fatigue, as measured by the Hoffmann reflex (H reflex) 58. A reduction in
motoneuron excitability, increased presynaptic inhibition of Ia afferents, or both, have
been correlated to a decreased H reflex 58.
Central fatigue has a large impact on motor control. If it arises from the cerebral
cortex, an impaired descending drive or reduced motivation is seen. Development of
central fatigue at the level of the spinal cord results in impaired alpha motor neuron firing
24
or suboptimal rate of recruitment to generate appropriate muscle force 25-27. Central
fatigue from impaired alpha motor neuron firing can lead to either loss of recruitment or
synergistic activation of multiple muscles 27. For repetitive tasks such as gait, the rate
slows and the time it takes to complete the task increases with fatigue 28.
Peripheral fatigue
During higher-intensity, short duration activities, fatigue will primarily occur
peripherally. Peripheral fatigue is correlated to the capability of a muscle to perform
physical work. As this fatigue occurs, there is impairment to the normal functionality of
the nerves and the muscles that are contracting, which translates to a decline in the
muscle’s ability to generate force due to the incapability of the body to meet the
increased energy demand in the contracting muscles. The most often suggested
mechanisms of peripheral fatigue consist of muscle fiber type and its distribution, the
readily available energy supply, the length of the muscle, and the muscle’s strength prior
to fatigue 22-24.
While both central and peripheral fatigue take place during sustained muscle
activity in which there is maximal exertion, during sub-maximal exertion, more
peripheral fatigue is occurring then central fatigue 29-31. In the case of a soccer game, it is
presumable that while both central and peripheral fatigue will occur for an individual
during a game, peripheral fatigue will be a greater factor due to the inherent quick bursts
of activity followed by periods of recovery.
Fatigue and sports
The majority of reported ACL injuries that occur in team sports are said to occur
towards the end of the game, indicating that fatigue may enhance non-contact ACL injury
25
risk 59, 60. Fatigue can influence the muscular mechanisms of the lower extremities,
resulting in kinematic changes when comparing fatigue to non-fatigue conditions 61.
Mechanics can be largely impacted by fatigue due to its effect on neuromuscular
function. Decreased neuromuscular function can result in decreased shifts in mechanical
energy between eccentric and concentric muscle contractions and delayed muscle
reaction 62. Fatigue has also been regarded as a primary component affecting the
musculoskeletal system as a result of its connection to diminished proprioception and
greater joint laxity 32 and the subsequent reduction in the skeletal muscle’s ability to
generate force 28, 33.
Previous studies looking at the effect of fatigue on various sports tasks have
utilized different protocols to simulate game fatigue. This has led to inconsistent findings
regarding joint dynamics during these tasks. The variation in fatigue protocols likely
contributes to the incongruous mechanical observations of the knee joint 35.
In a study looking at the effects of fatigue on side-cutting, neuromuscular activity
of the hamstrings decreased significantly post-fatigue. The observed decrease in
hamstring electromyography (EMG) amplitude represented a modified motor pattern
and/or diminished motor unit synchronization. Modification of a motor pattern can
impact optimal knee joint stability while diminished motor unit synchronization is a
strategy that utilizes muscle antagonist inhibition in order to offset agonist fatigue 63.
Sanna & O’Connor postulated that transverse plane kinetics affected by fatigue could
signal possible changes in lower extremity control strategies in order to be able to
complete the cutting task. The observed increased knee range of motion in the transverse
26
plane could indicate that injuries during a game may be occurring as a result of these
fatigue related kinetic changes 36.
The two main types of fatigue protocols used to study the effect of fatigue on
neuromuscular control strategies are short-term and long-term 36, 37. Short-term protocols
used to stimulate fatigue include a series of explosive tasks including vertical jumps and
sprints, squats and jumps, and single leg squats, whereas long-term protocols have
utilized 60 minute shuttle runs. In a study by Lucci et al. (2011), both short-term and
long-term fatigue protocols elicited altered hip and knee kinetic and kinematics during
sidestep cutting in female soccer players, and, after only five minutes of performing
either protocol, mechanical changes in the lower extremity took place that could enhance
the athletes risk for injury.
In an evaluation of the effect of fatigue on single limb landing, the results
indicated that there was not an overall effect of fatigue on valgus knee angles during
anticipated landing tasks. However, when the direction of the landing task was
unanticipated, there was a significant increase in knee valgus angles. This may have
occurred due to the effect of fatigue on coordination and timing. These results suggest
that there might be an increased risk for ACL injury when both fatigue and decisionmaking conditions are present 64.
Sports such as soccer and field hockey involve intermittent exercise with high
intensity activities such as jumping, turning, cutting, running and sprinting 65-67.
Research looking at the physiological components such as heart rate and metabolic
measures from blood and muscle samples obtained during competition have found that
27
athletes in these sports experience high aerobic work and anaerobic work during
competition 65-68. Initially, the capacity of these athletes was evaluated based on
continuous exercise tests (e.g. the Leger shuttle-run test, VO2max test). Development of
the Yo-Yo Intermittent recovery (IR) tests were in response to the lack of relevance of
continuous exercise tests for athletes participating in intermittent sports. The Yo-Yo IR
tests are comprised of 2x20m shuttle runs with increasing speeds, followed by a 10
second recovery period and the participant continues until they can no longer maintain
the speed and cover the distance in the allotted time. The Level 1 (Yo-Yo IR1) test begins
at a slower speed and has more modest increases in speed then the level 2 (Yo-Yo IR2)
test. For a less trained individual, the Yo-Yo IR1 test assesses the athlete’s ability to
complete a repetitive intense exercise bout with a large contribution of anaerobic work.
Several studies have observed the possible relationship between performance
during competition and performance of the Yo-Yo Intermittent Recovery tests 69-71.
Specific to soccer, there was a significant correlation between Yo-Yo Intermittent
Recovery 1 performance and the total amount of high intensity exercise for professional
players during a match 66, 68, 70. Research by Krustrup et al., found that amount of high
intensity running completed at the end of each match half was significantly correlated to
the Yo-Yo IR1 performance by elite female soccer players 70. The reliability and validity
are well supported and strong correlations have been made between the performance of
the Yo-Yo test and the amount of high intensity running during a soccer match, unlike
other testing methods such as repeated sprint tests, the Leger multistage fitness test, and
V02max testing. Krustrup et al, determined that performance of the Yo-Yo IR1 test was
the same when they repeated the test within a week 72. Research by Thomas et al.
28
assessed the test-retest reliability with 16 recreationally active subjects and attained a
correlation coefficient of 0.95 (p< 0.01) and the coefficient of variation being 8.7% 73.
General Anatomy of the Knee
The knee is a hinge joint formed by the femur and tibia. The patella is a sesamoid
bone that protects the anterior side of the joint. The patellar tendon originates from the
inferior portion of the patella and inserts on the tibial turbercle. The quadriceps muscle
group causes knee joint extension and is made up of the rectus femoris, the vastus
medialis, the vastus lateralis, and the vastus intermedius. This muscle group serves as an
antagonist to the anterior crucial ligament (ACL) and can decrease posterior subluxation
if a posterior cruciate ligament (PCL) injury occurs. The hamstring muscle group, made
up of the biceps femoris, semimembrinosus, and semitendinosus, acts as antagonists
medially and laterally to the PCL decreasing anterior subluxation.
Articular cartilage pads the joint, covering the ends of the femur and tibia and also
the posterior side of the patella. The medial and lateral menisci are cartilage pads that
function in load bearing, controlling rotation, and stabilizing translation. The collateral
ligaments are located on the lateral and medial sides of the joint stabilizing the knee by
limiting frontal plane motion. The posterior cruciate ligament prevents excessive
posterior translation of the tibia. The ACL originates on the posterior side of the
intercondylar notch and inserts on the anterior side of the intercondylar eminence. The
ACL acts in preventing hyperextension and anterior translation and it guides tibial
rotation as the knee extends 74.
29
General Ankle Anatomy
The ankle is comprised of three joints including the talocrural joint, the subtalar
joint and the inferior tibiofibular joint. Subtalar and talocrural joint motions include
inversion/eversion, dorsiflexion/plantarflexion, and abduction/adduction, or pronation
(dorsiflexion, abduction, and eversion) and supination (plantarflexion,adduction, and
inversion). The primary bones that make up the ankle include the distal tibia, fibula, and
talus. The tibia is the bone that bears the greatest amount of weight in the leg. The distal
articular surface of the tibia forms the top of the ankle mortise, and the medial malleolus
forms the medial border of the mortise and is the attachment site for the deltoid
ligaments. The fibula is a long, thin bone lateral to the tibia and is a site for both
ligamentous and muscular origin and attachment and creates lateral stability for the ankle
mortise. The lateral malleolus is an attachment site for the lateral ligaments of the ankle
and limits eversion, while the medial malleolus limits inversion. The lateral side of the
ankle is a more prevalent site for sprains which can lead to ligamentous avulsion from the
lateral malleolus when the ankle is inverted. The talus articulates with the distal tibia and
its medial and lateral borders articulate with the medial and lateral malleoli.
The talocrural joint is a modified synovial hinge joint formed by the articulation
between the talus, tibia, and fibula. It is surrounded by a joint capsule. If any of the
ligaments of the ankle are torn, it generally results in harm to the joint capsule as well as
irritation of the synovial lining. The only ligament that is an exception is the
calcaneofibular ligament, which exists extracapsularly. The three ligaments that exist on
the lateral portion of the ankle providing support to the talocrural joint are the anterior
talofibular ligament, the calcaneofibular ligament, and the posterior talofibular ligament.
30
The four medial ankle ligaments form the deltoid ligament that supports the medial
aspect of the ankle. The four ligaments are: the anterior tibiotalar ligament, the
tibiocalcaneal ligament, the posterior tibiotalar ligament, and the tibionavicular ligament.
The syndesmosis joint is comprised of the convex facet on the fibula which is buffered
from the concave tibial facet by dense fatty tissue. The inferior anterior and posterior
tibiofibular ligaments, and the crural interiosseous ligament maintain the syndesmosis.
The interosseous membrane is a strong fibrous tissue that fixes the fibula to the tibia and
is a point of origin for many muscles acting on the ankle and foot. On the proximal side,
an opening creates a passage way for the deep peroneal nerve and anterior tibial artery.
The distal side blends into the anterior and posterior tibiofibular ligaments creating
support for the distal tibiofibular syndesmosis joint.
The anterior compartment dorsiflexor muscles include the tibialis anterior, the
extensor halluces longus, the extensor digitorum longus, and the peroneus tertius. The
tibialis anterior is the prime ankle dorsiflexor and supinator. The extensor hallucis longus
aides in supination while the extensor digitorum longus assists with pronation. The
peroneus tertius is parallel to the fifth tendon of the extensor hallucis longus and it aides
in pronation. Across the anterior portion of the ankle mortise is the extensor retinaculum
which functions in securing the distal tendons of the muscles of the anterior compartment
as they cross the talocrural joint.
The lateral compartment structures include the peroneus longus and the peroneus
brevis. These muscles contribute mainly to eversion and also to plantarflexion. The
31
peroneal tendons are held in position posteriorly by the superior and inferior peroneal
retinaculum.
The posterior compartment structures are divided superficially and deeply. The
superficial posterior structures include: the triceps surae muscle group, made up of the
gastrocnemius and soleus, and the plantaris. The gastrocnemius and plantaris originate
on the posterior aspect of the femoral condyles while the soleus originates off the
posterior tibia. All three muscles insert on the calcaneus via the Achilles tendon. The
gastrocnemius and soleus are the primary movers during plantarflexion. The deep
posterior compartment structures include the flexor digitorum longus and the flexor
hallucis longus which flex the toes and assist in plantarflexion and inversion of the ankle.
The subtendinous calcaneal bursa is between the Achilles tendon and the calcaneus and
acts to decrease friction between these two structures. Located between the posterior
aspect of the Achilles tendon and the skin is the subcutaneous calcaneal bursa which
protects the tendon from trauma and decreases friction.
General Hip Anatomy
The acetabulofemoral joint is a joint formed by the femur and acetabulum on the
lateral aspect of the pelvis. The superior wall of the acetabulum is made by the ilium,
while the inferior wall is created by the ischium, and the medial wall by the pubis.
Centered within the fossa is a depression for the ligamentum teres. The outer rim of the
acetabulum is lined by a thick ring of fibrocartilage called the labrum. The femoral head
is round with its articular surface covered thickly by hyaline cartilage except for a central
depression where the ligamentum teres attaches. The head of the femur is connected by
32
the femoral neck and shaft, and the head is angled at approximately 125 degrees in the
frontal plane. This is referred to as the angle of inclination.
The iliofemoral ligament originates from the anterior inferior iliac spine and splits
inserting on the distal aspect of the anterior intertrochanteric line and on the proximal
aspect of the anterior intertrochanteric line and the femoral neck. This ligament
reinforces the anterior portion of the joint capsule, limiting extension, adduction, and
abduction and allowing individuals to stand upright with minimal use of muscles. The
pubofemoral ligament originates from the pubic ramus and inserts on the anterior aspect
of the intertrochanteric fossa, limiting abduction and hyperextension of the hip. The
ligamentum teres serves as a pass way for the artery of the ligamentum teres and does
little in terms of hip stabilization but provides the primary blood supply to the head of the
femur. The inguinal ligament originates on the anterior superior iliac spine and inserts on
the pubic symphysis. This ligament holds the soft tissues as they move anteriorly from
the trunk to the lower extremity.
Hip joint motion is controlled by groups of large extrinsic and small intrinsic
muscles. The larger muscle groups flex, extend, and internally rotate the joint. The
smaller intrinsic muscles externally rotate the hip. During cutting and running, the
abductors and adductors stabilize the hip. The anterior muscles include the rectus
femoris, the sartorius, and the iliopsoas group (psoas major, psoas minor, and iliacus).
The medial musculature includes the adductor longus, adductor magnus, adductor brevis,
the pectineus, and the gracilis. The lateral muscles include the gluteus medius, the tensor
fasciae latae, the piriformis, quadratus femoris, obturator internus, obturator externus,
33
gemellus superior, and gemellus inferior. The posterior muscles include the gluteus
maximus and the hamstring muscle group.
There are four main bursae located in the hip and pelvic region that function to
reduce friction between the gluteus maximus and adjacent bony structures. These bursae
include: the trochanteric bursa; the gluteofemoral bursa; the ischial bursa; and the
iliopsoas bursa.
Gender
In comparison to males, female athletes are four to six times more prone to ACL
injury 46. Due to the increased participation of females in sports, it is important to
examine the potential factors that contribute to the increased risk of injury. Previous
research has identified potential anatomical, hormonal, and neuromuscular differences
that may be attributed to the disparity in injury rates between males and females with
regards to the knee 75.
When performing the sidestep cutting maneuver, women have been found to have
decreased hip flexion and abduction, and larger internal rotation when compared to men
47
. Further, they have demonstrated less knee flexion and greater knee internal rotation,
larger knee abduction and pronation of the posterior foot. Research by McLean, Huang,
and van den Bogert found several gender differences when looking at lower extremity
posture at contact and peak knee valgus moment during sidestep cutting. The researchers
discovered that women displayed greater normalized peak valgus moments in the stance
phase of sidecutting and peak valgus moment is more responsive to initial contact hip
internal rotation and knee valgus excursions in women, which may indicate a greater
potential for injury 5.
34
Sigward and Powers found gender differences during side-step cutting. The
researchers identified women as having smaller sagittal plane moments and larger frontal
plane moments during the early deceleration phase. During early deceleration, the
women displayed a valgus moment during while the men had a varus moment. Overall,
the women displayed peak frontal moments relative to body mass that were two times
larger than the male subjects. The authors noted that female subjects had a valgus torque
at the knee while the male subjects had a varus torque on the knee 76. These findings are
consistent with those determined by McLean, Huang, and van den Bogert 5. Previous
studies utilizing both modeling and in vitro techniques have established that valgus
torque on the knee enhances the load placed on the ACL, especially at shallow knee
flexion angles such as those during the early deceleration phase, placing women at
greater risk for injury 9, 10.
Pollard, Heirdescheit, van Emmerik, and Hamill, found that women displayed less
coordination variability during an unanticipated sidecutting task 48. As alluded to by
Hamill et al., and reduction in variability in intralimb couplings is potentially related to
more rigid coordination patterns, which could lead to increased risk of lower extremity
injury 20. The authors determined that the gender differences in lower extremity
coordination variability predominantly observed in the initial loading phase of an
unanticipated cutting maneuver might attribute to the greater incidence of noncontact
ACL injuries in women. Further, it was suggested that these inflexible coordination
patterns in women during a competitive game may hinder their ability to properly adjust
to the frequent external perturbations common during sports play resulting in injury 48.
Based on these previous findings, it is possible that dampened lower extremity
35
coordination variability when coupled with fatigue during a game, could potentially lead
to an even greater likelihood of injury.
Anatomical Differences
There are several different hypotheses regarding sex related anatomical
differences that would potentially contribute to increased knee injury rates in females.
The first theory is centered around the difference in Q-angle between sexes. The Q-angle
is the angle formed by the femur from the hip joint to the knee joint. Despite increased
Q-angle in females, no association was found between a larger Q-angle and ACL injury
74
. The second theory indicates that the smaller femoral notch width relative to the size
of the ACL predisposes women to ACL injury. Despite this assertion, research regarding
this is contradictory 74. The final theory suggests that a narrow intercondylar notch leads
to a small ACL, increasing the risk of the knee to ACL injury, though research
surrounding this notion is contradictory as well 75.
Hormonal Differences
Sex steroid hormones and the menstrual cycle have been considered as risk
factors for the disproportionately greater incidence of non-contact ACL injuries in female
athletes compared to men 77, 78. Several studies conducted by Shultz et al., 79-81 have
determined that joint laxity in the knee fluctuates across the menstrual cycle and
correlates to absolute concentrations of sex steroid hormones. Although there is
conflicting research, some investigators have noted that a greater number of non-contact
ACL injuries occur around the late follicular phase, or ovulation, when there is a steep
increase in the level of estradiol, as opposed to the early follicular and luteal phases of the
cycle 77, 78, 82. Studies have indicated that estrogen binds to receptors on the ACL,
increasing its laxity. This increased laxity could possibly account for injury rate
36
differences between sexes. Males tend to have less laxity in the anterior portion of the
knee along with greater knee flexor strength. Estrogen has also been found to have a
large effect on muscle function, the strength of the tendons and ligaments, and the central
nervous system. A marked decrease in motor skills has been recognized during the week
prior to menstruation 75.
Neuromuscular Differences
Neuromuscular activation of the hamstrings and quadriceps may facilitate
dynamic frontal-plane knee joint stability due to their abduction and/or adduction
moment arms 83. Two general types of neuromuscular activation tactics have been
suggested to reduce external loading of the knee during dynamic sports tasks 83, 84. The
first method utilizes selective activation of muscles that have the mechanical ability to
offset the external load, and the second method entails arbitrary co-contraction without
discrimination due to mechanical advantage. Previous studies indicate that women may
utilize a selective activation method that supports abduction loading, which can be a
cause of ACL injury. Women tend to activate the lateral quadriceps and hamstrings
while having little activation of the medial thigh muscular. Activation of the medial
thigh has been found to resist abduction loads, thus, the method used by women might
potentially lead to injury 82, 85-87.
Lower extremity muscle activation differences during side-step cutting have been
observed between sexes. In a study by Hanson, Padua, Blackburn, Prentice, and Hirth,
the researchers found that female soccer players display a quadriceps dominant muscle
activation pattern during both a running-approach side-step cut and box-jump side-step
cutting maneuver in comparison to male soccer players, placing them at a greater risk of
37
injury 88. Also noted in the study was the greater Gluteus medius activation in female
subjects during the first half of the initial stance phase in sidecutting as well as greater
vastus lateralis activation during sidecutting then the male subjects. Research has found
that contraction of the quadriceps increases loading of the ACL unless the contraction of
the hamstrings is large enough to offset the quadriceps muscle contraction 89.
General Mechanisms of ACL Injury
The two primary classifications for anterior cruciate ligament (ACL) injury
mechanisms are contact and noncontact. A noncontact ACL injury refers to those
sustained when the player has no physical contact with another player, the ball, or another
object other than the ground. Previous studies note that 70 to 84% of all ACL injuries are
non-contact in nature 11, 46. The majority of these injuries occur during changes in
direction, rapid deceleration while running, cutting maneuvers combined with
deceleration, landing after a jump with the knee in or close to full extension, and pivoting
on a planted foot with shallow knee flexion 90, 91. The major components involved in the
aforementioned conditions include knee valgus, varus, internal rotation, external rotation
moments, and force being anteriorly translated 92.
A study by Zebis, et al., found that one of the most frequently reported noncontact ACL injury mechanisms is the plant-and-cut maneuver which leads to forceful
knee joint valgus and either internal or external tibial rotation with a very small knee
flexion angle. During these tasks, it appears that multi-planar knee loading (particularly
loading in the transverse and frontal planes) causes an exceedingly large quadriceps
contraction without the necessary magnitude of hamstring co-contraction 11.
38
When looking closer at knee flexion angles, Nagano, Ida, Akai, and Fukubayashi,
determined that knee flexion angles less than 30 degrees resulted in a large strain force on
the ACL caused by quadriceps contraction, especially during unilateral landing as a
means to prevent falling 93. Loading of the quadriceps can lead to anterior tibial
displacement, knee internal rotation, and knee valgus motions.
During rapid deceleration, large quadriceps contractions at small knee flexion
angles place individuals at a greater risk of injuring the knee. This has been observed
more so in female athletes as they tend to be quadriceps dominant in response to anterior
tibial translation, while male athletes tend to recruit the hamstrings to a greater degree
than the quadriceps. The hamstrings act to resist strain on the ACL, serving as an agonist
to the ACL, whereas the quadriceps act as antagonists, increasing strain on the ACL,
particularly when the knee flexion angle is small. It has also been noted that during these
non-contact injuries, the knee went into valgus while simultaneously rotating either
internally or externally. This was also found to occur in a hyper-extended knee joint
position from a shallow knee flexion angle of approximately five to twenty degrees. The
risk of the ACL sustaining larger anterior loads is greater near full knee extension when
the ACL is acting to prevent anterior translation of the tibia relative to the femur 7, 11, 51.
Conclusions
Though there are many studies that look at the effect of fatigue on lower
extremity mechanics during dynamic sports tasks, there is a lack of research regarding the
effect of unanticipated actions coupled with fatigue. Due to the nature of sports, it is
more realistic to consider that most athletes will not be able to pre-plan these dynamic
tasks during a game. Thus, it is important to increase the ecological validity and practical
39
implications of the research in this way. Since it is during these unanticipated tasks
towards the end of a game that most non-contact injuries occur, it is important to
investigate how fatigue impacts mechanics in these instances. Previous studies utilizing
anticipated tasks did not observe significant mechanics differences between pre and post
fatigue conditions. Furthermore, there is a lack of research looking at the kinetic and
kinematic changes that occur across the ankle, knee, and hip during unanticipated and/or
fatigue conditions. There is also a lack of research examining differences between limbs
during the sidecutting maneuver. By comparing the lower extremity kinetics of a
sidecutting maneuver both pre and post fatigue, it may be possible to isolate specific
changes that may place individuals at a greater risk of injury. The purpose of this study is
to investigate the effect of fatigue on lower extremity mechanics during the unanticipated
cutting maneuver. The secondary purpose of this research is to examine differences
between limbs with respect to limb dominance.
40
CHAPTER 3: Manuscript
THE EFFECT OF FATIGUE ON
LOWER EXTREMITY MECHANICS
DURING THE UNANTICIPATED
SIDECUTTING MANEUVER
Kaitlyn J. Weiss, Henry Wang, Dorice Hankemeier, D. Clark Dickin
School of Physical Education, Sport, and Exercise Science,
Ball State University,
Muncie, IN 47306, USA
Address Correspondence to:
D. Clark Dickin
Biomechanics Laboratory
Ball State University
McKinley Avenue, PL 202
Muncie, IN 47306
Phone: (Int+1) (765) 285-5178
Fax: (Int+1) (765) 285-8762
Email: dcdickin@bsu.edu
Abstract
Fatigue has been observed to affect lower extremity mechanics during the cutting
maneuver. However, there is a lack of research examining the effect of fatigue and limb
dominance on lower extremity mechanics during unanticipated sidecutting. Objectives:
This research sought to assess mechanical differences pre- and post-fatigue and with
respect to limb dominance. Design: Repeated measures. Methods: Thirteen female
collegiate soccer and field hockey players performed right and left unanticipated
sidecutting following the Yo-Yo Intermittent Recovery test (Yo-Yo IR), a two minute
treadmill run at a predicted VO2max, and maximum vertical jumps. Mechanical measures
of ankle, knee, and hip motion were obtained during the stance phase of the cut.
Repeated measures 2x2 ANOVAs were performed to look at fatigue and limb
differences. Alpha level set a priori at 0.05. Results: At initial contact and peak stance,
significant changes pre- to post-fatigue were observed. At initial contact there was a
reduction in knee flexion angles along with increased ankle dorsiflexion angles postfatigue. At peak stance: increased knee adductor moments post-fatigue; greater ankle
eversion moments on the dominant limb (DL) as well as increased eversion moments
post-fatigue for both limbs. There was a differential effect of fatigue on peak hip
abduction angles and hip internal rotation angles at initial contact which were altered in
the DL only; decreased hip adductor moments occurred post-fatigue as well as decreased
power absorption. Conclusions: Results from this study indicate that lower extremity
mechanics are altered as an effect of fatigue such that injury risk may be elevated.
Keywords: Biomechanics; Limb Dominance; Anticipation
42
Introduction
The sidecutting maneuver is a dynamic sports task that allows the performer to
change direction from standing, walking, or running. Improper mechanical execution of
the sidecut can place the ligaments of the knee at the greatest risk of injury.36 Given that
the majority of dynamic cutting tasks are not pre-planned during games, anticipated
maneuvers are likely not a true reflection of lower extremity mechanics.
The use of unanticipated dynamic tasks such as a cutting maneuver is meant to
mimic the nature of the task’s performance in game situations.16 Ilmane and LaRue1
discovered that temporal constraints (self-initiated, anticipation-coincidence, and reaction
condition) have a significant effect on anticipatory postural adjustments. These
anticipatory postural adjustments are centrally produced as a feed-forward mechanism to
offset the mechanical effects of predicted perturbations on stability in dynamic sports
tasks.19 Thus, during an unanticipated task an individual’s ability to adjust to the
environmental perturbations commonly experienced during a game or practice may be
restricted, making them more susceptible to potential injury. Research looking at the
differences between unanticipated and anticipated lower extremity biomechanics during
sidecutting determined that the unanticipated condition displayed greater knee abduction
and internal rotation, hip abduction, and decreased knee flexion.16
The majority of reported ACL injuries in team sports occur towards the end of the
game, indicating that fatigue may enhance non-contact ACL injury risk.59 Fatigue can
influence the muscular mechanisms of the lower extremities, resulting in kinematic
changes when compared to non-fatigue conditions. 61 Research has demonstrated that
changes in transverse plane kinetics could be indicative of injury mechanisms occurring
43
as a result of these fatigue-related changes.36 In an evaluation of fatigue on single limb
landing, increases in knee valgus angles were reported when landing direction was
unanticipated, and may have been due to the effect of fatigue on coordination and timing.
These results suggest that ACL injury risk may increase when both fatigue and decisionmaking conditions are present.64
When performing the sidecut, women have been found to have kinematic and
kinetic differences that increase their risk of injury four-to-six times above their male
counterparts.46 These differences include: decreased hip flexion and abduction, larger hip
internal rotation, less knee flexion, greater knee internal rotation and abduction, pronation
of the plant foot, and greater normalized peak valgus moments during the stance phase.47
Studies utilizing both modeling and in vitro techniques have established that valgus
torque on the knee enhances the load placed on the ACL, especially at shallow knee
flexion angles such as those during the early deceleration phase, placing women at
greater risk for injury.7, 9-11 Female athletes have also been observed as having a
reduction in their performance of the sidestep cut when it was unanticipated, which could
be attributed to less coordination variability during an unanticipated sidecut compared to
men. It has been suggested that these inflexible coordination patterns during a
competitive game may hinder females ability to properly adjust to the frequent external
perturbations common during sports play, resulting in injury.48 Based on these previous
findings, it is important to study the effect of fatigue on unanticipated sidecutting as
dampened lower extremity coordination variability when coupled with fatigue during a
game, could further exacerbate injury risk.
44
Previous research has also assessed lower-limb dominance related to the risk of
ACL injury and athletic performance.41, 42 Though significance between limbs has not
been found, limb asymmetries may still pre-dispose athletes to injury.41, 42, 94 Contrasting
biomechanical variables between dominant and non-dominant limbs during single leg
drop landing, suggest that the dominant limb demonstrated greater thigh strength, greater
knee flexion range of motion, and enhanced proprioception in an athletic population.
Thus, by identifying kinetic and kinematic differences pre- and post-fatigue between
limbs may enhance our knowledge and abilities to design preventative techniques for
training, rehabilitation, and conditioning.45
Though several studies have looked at the effect of fatigue on lower extremity
mechanics during dynamic sports tasks, few have observed its effects on unanticipated
actions and limb dominance. Although previous studies utilizing anticipated tasks have
not observed differences due to fatigue, it is more realistic to consider that most athletes
will not be able to pre-plan these dynamic tasks during a game. Since unanticipated
cutting towards the end of a game are accompanied by increased risk of injury, it is
important to investigate how fatigue impacts mechanics in these instances. Furthermore,
there is a lack of research assessing kinematic and kinetic changes occurring across the
ankle, knee, and hip during unanticipated and/or fatigue conditions. Given that the body
functions most efficiently when its joints are aligned, any deviations may result in
compensatory mechanisms to re-establish equilibrium. Evaluating all three joints
provides a better depiction of lower extremity mechanics and how all three joints are
affected by fatigue.
45
The primary purpose of this study to investigate the effect of fatigue on lower
extremity mechanics during the unanticipated sidecut. The secondary purpose is to
evaluate differences between the dominant and non-dominant limbs. We hypothesize
that fatigue will alter lower extremity kinetics and kinematics such that: the knee will
experience greater external loads and more shallow knee flexion angles; the hip will have
decreased flexion and abduction, and larger internal rotation; and the ankle will exhibit
greater inversion.
46
Methods
Thirteen college-aged NCAA Division I female soccer and field hockey players
volunteered for this study (mean age = 20.31±1.84years; height = 1.68±5.7m; mass =
61.99±6.45kg). A power analysis was performed to confirm the sample size with 80%
statistical power with an alpha level of 0.05.52, 76 Participants were excluded if they
reported any of the following: a history of ACL injury or surgery; history of knee injury
that may have resulted in joint laxity (e.g., PCL, MCL, or LCL injury); musculoskeletal
injuries in the previous six months that prevented them from participating in activity
(practices and games); or any physical or neurological condition that impaired their
ability to complete the task required. A health history questionnaire was used to assess
health status, and all participants signed a university approved informed consent
document.
Three-dimensional kinematic and kinetic data was collected with a passive, 12camera (F40) Vicon motion-analysis system (VICON, Oxford Metric Ltd., Oxford, UK)
sampling at 240 Hz. Spherical retro-reflective markers (14mm and 25mm) were placed
on specific anatomical landmarks following a modified Plug-in Gait model.95, 96 Ground
reaction forces were collected using two AMTI force platforms (Model OR6-7-2000,
Advanced Mechanical Technologies Inc., Watertown, MA, USA) embedded within the
testing floor sampling at 2400 Hz.
Participants came to the Ball State University Biomechanics Laboratory for a
single 90 minutes testing session. All individuals were outfitted in compression clothing,
standardized indoor soccer footwear, and had anthropometric measures taken. Using a
47
modified plug-in-gait model, markers were placed on anatomical landmarks of both the
upper and lower body that included 4-marker clusters on each thigh and shank. The
dominant limb (DL) was defined as the leg used to kick a ball, and the opposite limb was
defined as the non-dominant limb (NL) 41. All participants identified their DL as the
right limb. Participants performed a self-selected dynamic warm up for ten minutes,
followed by three vertical jumps to determine maximum jump height. Participants
practiced at least three of each cutting task (i.e., cut left, cut right and stop) or until they
felt comfortable. Timing gates were set up 3m from the center of the force plates and as
participants ran through the timing gates, custom built computer software randomly
generated a projection of the dynamic task onto a screen in front of the participants i.e.,
direction arrows or a stop sign . Tape lines were placed
to the right and left of the
plate to guide the angle of the cuts. The pre-fatigue dynamic task trials were then
performed, with 45-second rest period between pre-fatigue trials to minimize fatigue.40
The pre-fatigue portion of the testing was terminated once the participant had completed
four good right and four left cutting trials, consisting of placement of the stance foot on
the force plate and then a
cut in the direction contralateral to the stance foot.6
After completing the pre-fatigue dynamic task trials, participants performed the
YYIRT.69 This protocol consisted of repeated high intensity 20 meter shuttle runs
starting at 10 km/h and increasing on successive trials by 0.5 km/h with 10 seconds of
recovery after each trial (20m x 2), and was repeated until the athlete was unable to
successfully complete two 20m sprints in the allotted time. This test has been proven to
be valid and reliable with ~5% variation between tests 72 and has been correlated to
match play in elite male soccer players.69 The fatigue testing was conducted in a
48
gymnasium next to the laboratory. To ensure that participants maintained their fatigued
state once back in the laboratory, they ran for two minutes on the treadmill in the testing
area at their estimated VO2max speed as calculated by the YYIRT. Immediately
following the treadmill task, participants performed vertical jumps until unable to reach
80% of their maximum vertical jump height for three successive jumps. Finally,
participants performed the post-fatigue randomized dynamic tasks trials, without a rest
period between each trial. The post-fatigue trials were considered complete once four
good right and left cuts had been performed.
The raw marker trajectory data was reconstructed in Nexus (VICON, Oxford
Metric Ltd., Oxford, UK) and further processed in Visual3D (C-Motion, Germantown,
MD, USA) with the use of standard segment and joint definitions. Net joint moments
were calculated using standard inverse dynamics equations. Raw 3D coordinate data for
markers were filtered using a fourth-order Butterworth filter with a cutoff frequency of 8
Hz which is consistent with previous studies.8, 51 Three-dimensional knee angles were
calculated using a joint coordinate system approach. Repeated measures 2x2 (time x
leg ANOVA’s were performed to look at: ankle, knee, and hip angles in all three planes
at initial contact; peak ankle, knee and hip angles and moments in the frontal plane, and
peak power absorption over the entire stance phase. The alpha level was set at 0.05 and
analyses were performed using SPSS version 19.0 (SPSS Inc., Chicago, IL, USA).
Results
The descriptive statistics for kinetic and kinematic data are in Tables 1 and 2. On
average, the participants completed the YYIRT at level 17 (SD 5.4) at approximately
49
96% of their predicted maximum heart rate. Dependent t-tests across all cutting trials
confirmed there were no significant differences p≥.0
in cutting speed between pre- and
post-fatigue and between limbs.
We observed significant changes pre- to post-fatigue when collapsed across
limbs. The knee experienced decreased flexion angles at initial contact (F1,11= 37.25,
p≤0.001, eta2= 0.77), and increased peak adductor moments (F1,13= 5.58, p=0.034, eta2=
0.3). The ankle displayed increased dorsiflexion angles (F1,11= 5.08, p=0.046, eta2= 0.5)
at initial contact, and increased peak eversion moments (F1,11= 7.34, p=0.019, eta2= 0.38).
The hip displayed decreased flexion angles (F1,11= 21.06, p=0.001, eta2= 0.66) at initial
contact, decreased peak adductor moments (F1,11= 9.104, p=0.011, eta2= 0.431) and
decreased hip power absorption (F1,11= 5.2, p=0.042, eta2= 0.3).
Significant differences were only observed for the ankle between limbs when
collapsed across time. Greater peak ankle eversion moments were noted on the DL
(F1,11= 6.7, p=0.024, eta2= 0.36) and the NL exhibited greater ankle power absorption
(F1,11= 12.15, p=0.004, eta2= 0.5).
There were significant interaction effects for peak hip abduction angles (F1,11=
6.75, p=0.023, eta2= 0.36) and for hip internal rotation angles for limb and time (F1,11=
6.36, p=0.028, eta2= 0.63), however no significant simple main effects were found for
either limb or time on the two measures.
Discussion
The purpose of this study was to investigate the effect of fatigue on lower
extremity mechanics during the unanticipated sidecut and to evaluate differences between
50
limbs. Our main findings determined that participants performed the sidecut with a more
vertical lower limb post-fatigue, which is consistent with previous research.34, 40 These
findings are consistent with the current literature indicating fatigue alters mechanics such
that the knee is placed in a position associated with an increased injury risk.
With respect to the knee, we observed increased peak adductor moments
following fatigue, which have been strongly linked to injury risk.6, 34, 36 These increased
adductor moments coupled with anterior tibial force may increase ACL loading.9-11
During cutting, knee flexion functions in attenuating the increased loading during the
deceleration phase. It also lowers the athlete’s center of gravity, decreasing the moment
of inertia of their body about their foot and creating a more stable base of support during
the cut.52 The increased adductor moments coupled with decreased knee flexion angles
compromise the integrity of the knee joint, increasing injury risk due to the resulting
large quadriceps forces and insufficient co-contraction of the hamstrings. We also
observed increased peak ankle eversion moments which may have been a mechanical
strategy by the lower limb in conjunction with the increased peak knee adductor moments
as a means of stabilizing the tibia.
Increased dorsiflexion at initial contact may also have been a compensatory
mechanism enhancing ankle power absorption through the stretch reflex mechanism, and
in response to the decreased ability of the hip to absorb power post-fatigue. The finding
of decreased power absorption at the hip may be a result of neuromuscular fatigue. The
decreased hip adductor moments may have also been a compensatory mechanism
attempting to reduce external loading, thereby allowing fatigued hip adductor muscles to
produce less force to control hip motion. Decreased neuromuscular function can result in
51
decreased shifts in mechanical energy between eccentric and concentric muscle
contractions and delayed muscle reaction 62, and also as a primary component in
diminished proprioception and greater joint laxity 32 and ultimately a reduced ability to
generate force.28, 33 Decreased hip flexion at initial contact produces a more vertical
limb. This positions the foot closer to the midline of the body and results in decreased
gravitational moment of inertia acting on the body’s center of mass, and decreases the
need for large internal moments to be created by the hip musculature.
Anticipation of a movement can lead to altered reflex responses and postural
modifications in order to decrease the impending perturbation and uphold the necessary
posture.17 In the current study, the cutting maneuvers were unanticipated both pre- and
post-fatigue, which limited the time the participants had to perceive the stimulus and
coordinate lower extremity mechanics. It is possible that participants constrained
mechanics under both the fatigue and non-fatigue conditions, impacted variables such as
hip abduction, and ankle inversion, which were hypothesized to be altered by fatigue.
Though previous studies have not identified a significant relationship between
lower limb dominance and potential for injury, differences in mechanics between limbs
may indicate increased risk of injury.42 This study observed significant differences in
frontal plane peak ankle moments and powers. The significant interaction effects for hip
internal rotation angles at initial contact indicate that the DL was more influenced by
fatigue. The effect was also evidenced with decreased peak hip abduction and internal
rotation angles following fatigue that resulted in the DL more closely mimicking
mechanics seen in the NL. Though both limbs were affected by fatigue, possible
differences in motor control strategies may have impacted the DL’s ability to control hip
52
motion, leading to compensatory mechanisms to decrease external loading as a means to
prevent injury following fatigue. Though the DL was differentially affected by fatigue, it
appears that the NL is at a greater risk of injury given its mechanics both pre- and postfatigue, including decreased hip abduction and internal rotation angles. Previous research
has identified an increased trend towards injury to the left limb in female athletes which
is consistent with our findings as all 13 participants identified their left limb as the NL.2
The findings from this study provide new information pertaining to ACL injury
mechanisms in elite level female athletes. Although the findings help to improve our
understanding on this injury there are some limitations. The Yo-Yo Intermittent Recovery
Test (YYIRT) has been observed to produce similar physiological effects experienced
during a soccer match and thus may better simulate fatigue experienced during match
play.69 However, it may vary in its influence on the cutting maneuver due to the
significantly decreased performance time relative to actual game play. With the addition
of maximum vertical jump height and the 2-min treadmill run, we induced fatigue using
several methods in an attempt to ensure that the athlete maintained a fatigued state during
testing. Future research should include other sports, competitive levels, age groups, and
males to identify whether similar trends apply across different groups.
Conclusion
Prior research has determined that following fatigue, mechanical alterations result
in increased knee abduction and internal rotation angles, increased hip rotation angles,
increased hip internal rotation moments, and decreased knee flexion angles. The present
study sought to evaluate differences primarily in the frontal plane due to their
53
implications in injury risk. Our participants demonstrated increased peak knee moments
following fatigue in addition to a more vertical limb, which may have significantly
impacted loading of the ACL. Further, this study identified differences with respect to
limb dominance that may increase the risk of injury. This information may be beneficial
for clinicians, trainers, and coaches in establishing injury prevention strategies.
Intervention programs should focus on improving bilateral symmetry and kinematics by
teaching athletes to cut with a more flexed hip and knee to promote hamstring cocontraction and decreasing the moment of inertia of the body about the foot.
Practical Implications
. Post-fatigue, altered mechanics were observed that may place a female athlete at an
increased risk of injury.
. Preventative training focusing on proper mechanics in a fatigued state may be
beneficial.
. Training protocols should emphasize reactionary cutting drills that involve cutting off
both the dominant and non-dominant limbs.
Acknowledgments
The authors received no financial support for this research.
54
Tables and Figures
Table 1
Dominant
Limb
NonDominant
Limb
Pre-test
Mean
SD 95% CI
Posttest
Mean SD 95% CI
Pre-test
Mean
SD 95% CI
Posttest
Mean SD
95% CI
Initial Contact
Knee flexion
26.42
(+)/extension (-)
Knee adduction
-4.77
(+)/abduction (-)
Knee internal
5.26
(+)/external rotation ()
Ankle dorsiflexion
3.01
(+)/plantar flexion (-)
4.70 23.43,
22.86 6.28 18.86,
28.59
29.40
26.85
5.53 -8.28, -5.84 4.23 -8.53, -4.59
1.26
3.15
6.31 1.25, 9.27 5.50 6.49 1.38, 9.62 4.94
5.51 25.10,
24.48 5.37 21.07,
32.09
27.90
3.73 -6.97, -5.56 3.58 -7.83, 2.22
3.28
8.20 -.27, 10.14 4.89 6.18 .96, 8.82
4.73 .01, 6.01 3.72
7.63 -5.47, 4.23 2.30
Ankle inversion
15.18
(+)/eversion (-)
Ankle internal
-14.47
(+)/external rotation ()
Hip flexion
58.68
(+)/extension (-)
Hip adduction
-12.07
(+)/abduction (-)
Hip internal
5.95
(+)/external rotation ()
8.32 9.89,
20.47
7.37 -19.15,
-9.78
17.57 7.72 12.66,
22.47
-13.79 8.12 -18.95,
-8.63
6.08 54.81,
62.54
5.58 -15.61,
-8.52
7.12 1.42,
10.48
52.49 9.14 46.69,
57.68
58.30
-10.21 6.31 -14.23,
-11.99
-6.20
2.32 4.15 -.32, 4.95 3.42
7.49 52.92,
52.49 10.41 46.05,
62.44
58.93
6.78 -16.30,
-10.01 8.05 -15.13, -7.69
4.89
4.52 .55, 6.29 3.71 3.43 1.53, 5.89
7.07 -14.53,
-6.36
6.62 27.27,
35.27
6.21 -25.37,
-17.86
-10.74 6.44 -14.45,
-7.02
32.04 6.87 27.89,
36.19
-19.16 5.48 -22.47,
-15.85
5.03 -14.86,
-9.06
4.56 28.58,
34.08
7.90 -23.69,
-14.14
5.67 .12, 7.32 -0.62
16.81
-14.77
6.49 12.69,
20.94
5.87 -18.50,
-8.63
7.09 -2.21, 6.80
17.27 6.74 12.99,
21.55
-13.99 5.85 -17.70,
-10.28
Peak Stance
Knee adduction
(+)/abduction (-)
Ankle inversion
(+)/eversion (-)
Hip adduction
(+)/abduction (-)
-10.44
31.27
-21.62
-11.96
31.33
-18.91
-12.46 6.38 -16.14,
-8.77
32.17 5.09 29.09,
35.25
-18.72 6.49 -22.64,
-14.80
Descriptive statistics (mean, standard deviation) for kinematic variables between dominant and non-dominant limbs
pre- and post-fatigue at initial contact and peak stance. All variables are measured in degrees.
55
Table 2
Dominant
Limb
Pre-test
Peak Stance
Mean
Non-Dominant
Limb
Posttest
SD 95% CI Mean SD
-1.28, - -1.24 .68
.32
Pre-test
95% CI Mean
SD
Posttest
95% CI Mean SD
95% CI
-1.64, .85
-1.18
1.40 -1.99, - -1.50 .68
.38
-1.89, 1.11
Knee adduction
-.80
(+)/abduction (-) moment
.83
Knee power (-) absorption/ -20.45
(+) production
5.83 -23.97, -20.60 5.57 -23.97,
-16.93
-17.24
-21.33
5.65 -24.75, -21.42 4.95 -24.41,
-17.91
-18.43
Ankle inversion
.65
(+)/eversion (-) moment
Ankle power (-)
-9.51
absorption/ (+) production
.28
.50
.39
1.47 -10.40, -9.37 1.36 -10.19,
-8.62
-8.55
-11.04
2.09 -12.31, -11.43 3.18 -13.35,
-9.78
-9.51
Hip adduction
-1.87
(+)/abduction (-) moment
.79
-1.19 1.02 -1.82, .59
-1.63
1.53 -2.57, - -1.54 .89
.71
Hip power (-) absorption/ -6.58
(+) production
2.36 -8.01, - -5.29 2.76 -6.96, 5.15
3.62
-7.99
3.35 -10.02, - -6.09 2.86 -7.83, 5.97
4.37
.48, .82 .74
-2.36,
-1.40
.23
.60, .88
.26, .74 .61
.35
.39, .82
-2.09, 1.01
Descriptive statistics (mean, standard deviation) for kinetic variables between dominant and non-dominant limbs preand post-fatigue at peak stance. External moments measures in Nm/kg.
56
Figure 1. Interaction Effect of (Limb*Time) for Hip Internal Rotation Angle at Initial
Contact.
Figure 2. Interaction Effect of (Limb*Time) for Peak Hip Abduction Angle.
57
Chapter 4: Summary and Conclusions
The current study presents several limitations with regards to the fatigue protocol,
subject population, and testing set up. Though the Yo-Yo IR test has been determined to
produce similar physiological effects to that of game conditions, it may vary in its
influence on the cutting maneuver due to the significantly decreased performance time
relative to actual game play. To induce fatigue, the Yo-Yo IR test, maximum vertical
jump height, and heart rate were used and although the individual reported being fatigued
and showed decreased performance, further assessments of overall fatigue should be
employed. Additionally, only female, collegiate level soccer and field hockey players
were used in this study. Future research including a greater number of sports, multiple
skill levels, age groups, and both genders would enhance our understanding of the impact
of fatigue and the possible effects of limb dominance on injury risk. The lab set-up for
testing somewhat decreased the external validity of the study in that it does not reflect
true sporting conditions. All athletes tested primarily compete outdoors, meaning the
current study tested them on different playing surface conditions and in a restricted space.
The decreased space allotted for the cutting maneuver inhibited the participants from
reaching top running speeds prior to performing the cutting maneuver off the force plates.
The signaling images were created to induce a similar reactionary response to what the
participants would experience during a game, however, future studies may increase the
validity of this by using actual game footage, or a live opponent for the participants to
react to. The current study did not use electromyography during testing, so
neuromuscular fatigue/dysfunction was speculated. Future research may also choose to
test interventional training programs and re-test athletes upon completion of these
programs.
Prior research has determined that following fatigue, mechanical alterations result
in increased knee abduction and internal rotation angles, increased hip rotation angles,
increased hip internal rotation moments, and decreased knee flexion angles. The present
study sought to evaluate differences primarily in the frontal plane due to their
implications in injury risk. Our participants demonstrated increased peak knee moments
following fatigue in addition to a more vertical lower extremity posture, which may have
significantly impacted loading of the ACL. Further, this study sought to identify changes
across the ankle, knee, and hip as well as differences with respect to knee dominance in
order to enhance our current understanding of possible alterations that may increase the
risk of injury. As a result of this study, it was determined that fatigue impacts mechanics
across the ankle, knee, and hip, and together, these altered mechanics place the knee at a
greater risk of injury. This information may be beneficial for clinicians, trainers, and
coaches in establishing injury prevention strategies, with respect to fatigue and
contralateral limb asymmetries.
59
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