Athletic Training and Sports Health Care

Original Research 

Effects of Activity Type and Hip Fatigue on Knee Kinematics in Female and Male Athletes

Melisa A. Fazio, MS, ATC; Jennifer S. Howard, PhD, ATC; Carl G. Mattacola, PhD, ATC, FNATA; Timothy L. Uhl, PhD, PT, ATC, FNATA; Cale A. Jacobs, PhD, ATC

Abstract

Landing mechanics have been documented as a risk factor for knee injuries in female athletes. This article aimed to determine the effect of hip abductor/external rotator fatigue and activity type on landing kinematics. Selected hip and knee landing kinematics were measured before and after a hip fatigue task in 45 healthy participants regularly performing either multiplanar or uniplanar activities. Across conditions, the multiplanar activity male group demonstrated less knee abduction than the uniplanar (P = .001) and multiplanar (P = .018) activity female groups. In addition, the uniplanar activity female group demonstrated greater knee abduction than the multiplanar activity female group (P = .031). Hip adduction during landing increased postfatigue (P = .001); however, fatigue did not affect knee kinematics. Landing kinematics may depend on activity participation, as well as gender. Therefore, it is important to classify research participants based on the type of activity in which they regularly participate.

Abstract

Landing mechanics have been documented as a risk factor for knee injuries in female athletes. This article aimed to determine the effect of hip abductor/external rotator fatigue and activity type on landing kinematics. Selected hip and knee landing kinematics were measured before and after a hip fatigue task in 45 healthy participants regularly performing either multiplanar or uniplanar activities. Across conditions, the multiplanar activity male group demonstrated less knee abduction than the uniplanar (P = .001) and multiplanar (P = .018) activity female groups. In addition, the uniplanar activity female group demonstrated greater knee abduction than the multiplanar activity female group (P = .031). Hip adduction during landing increased postfatigue (P = .001); however, fatigue did not affect knee kinematics. Landing kinematics may depend on activity participation, as well as gender. Therefore, it is important to classify research participants based on the type of activity in which they regularly participate.

Ms Fazio is from the University of Colorado, Boulder, Colorado; Dr Howard, Dr Mattacola, and Dr Uhl are from the University of Kentucky, Lexington, Kentucky; Dr Jacobs is from ArthroMetrix LLC, and ERMI Medical Devices, LLC, Atlanta, Georgia.

This study was funded by the National Athletic Trainers Association Research and Education Foundation (#505MGP004), Dallas, Texas. The authors thank Robert Shapiro, PhD, for his contribution to the conception and design of this study.

The authors have no financial or proprietary interest in the materials presented herein.

Address correspondence to Melisa A. Fazio, MS, ATC, University of Colorado, Dal Ward Building, 100 Stadium Drive, 368 UCB, Boulder, CO; e-mail: Melisa.fazio@colorado.edu.

Received: December 20, 2010
Accepted: August 18, 2011
Posted Online: October 17, 2011

The role of the hamstrings and quadriceps in relation to anterior cruciate ligament (ACL) injuries has been extensively studied; however, the role of hip musculature is still relatively undefined.1–4 The hip musculature—including the gluteus minimus, medius, and maximus; obturator internus and externus; gemelli superior and inferior; tensor fasciae latae; and quadratus femoris—are the primary hip external rotators and abductors.5,6 The inability of these muscles to adequately control internal rotation and adduction of the femur during athletic maneuvers can result in excessive knee abduction. After a jump, women tend to land in this position more so than men,2,3,7 and this position has been identified as a predictor of ACL injury in women.8 It may be hypothesized that decreased hip external rotation or abduction force production, such as what occurs with fatigue, may result in exacerbated knee abduction on landing. However, the role of the hip musculature in controlling knee biomechanics during landing has not been clearly described.

As athletes fatigue, they encounter a decrease in proprioception and the ability to generate muscular force.9–11 Limited and conflicting data exist regarding the effects of fatigue on knee abduction during a jump landing.12–16 For example, Carcia et al,13 Gehring et al,14 Kernozek et al,15 and McLean et al16 reported no change in knee abduction, or knee valgus, angle immediately following various types of fatigue protocols, including leg press,14 parallel squats,15 plyometric bounding and step-ups,16 and hip abductor isometrics.13 However, Carcia et al13 reported a significant increase in knee valgus when participants performed landings following a recovery period after a fatigue protocol, and Benjaminse et al12 reported a decrease in knee abduction angle following a modified Astrand fatigue protocol. Of these studies, only Carcia et al13 used a fatigue protocol that specifically targeted the hip musculature.

Strength of the hip musculature in women may also affect knee position during landing. Women with lower eccentric peak hip abduction torque demonstrated higher knee valgus angles during single-leg jump landings.17 On the basis of the aforementioned studies, it is unclear whether hip musculature fatigue causes athletes to land in a position of hip adduction and internal rotation and knee abduction, placing them at risk of injury.

One factor that may influence hip strength and neuromuscular control is activity participation. Athletes who regularly participate in highly dynamic and multiplanar activities may have greater frontal and transverse plane hip strength and control, compared with athletes who participate in primarily uniplanar or sagittal plane activities. Despite the large amount of research focusing on kinematics of the knee, the influence of activity type on landing kinematics has not been explored, particularly in relation to fatigue of the hip abductors and external rotators. Although some of the research involving ACL risk factors, such as landing kinematics, focused on participants of only 1 sport (basketball, volleyball, soccer),3,7,16,18–20 some studies have not classified participants based on the type of activity in which they participate.14,21–24 Regular participation in multiplanar activities, such as jumping and cutting, may result in a training response that alters landing mechanics over time. Failure to distinguish between research participants who frequently perform jumping activities and those who do not may distort research results. A direct comparison of landing kinematics between athletes who participate in multiplanar activities and those who participate in uniplanar activities has not been presented previously in the literature.

The purpose of this study was to determine the effects of hip abductor and external rotator muscle fatigue on the frontal and transverse plane kinematics of the hip and knee in different athletic populations. The research hypotheses were:

  • Implementation of a hip musculature fatigue protocol would result in greater hip adduction, hip internal rotation, and knee abduction during a single-leg jump landing.
  • Uniplanar activity female athletes would demonstrate significantly greater hip adduction, hip internal rotation, and knee abduction in the postfatigue condition than both multiplanar activity male and female athletes in the prefatigue and postfatigue conditions.
  • Both the multiplanar and uniplanar activity female groups would demonstrate significantly greater hip adduction, hip internal rotation, and knee abduction than the multiplanar activity male group during a single-leg jump landing.
  • Hip abduction and external rotation peak torque would be significantly greater in the multiplanar activity male group than in both the multiplanar and the uniplanar activity female groups.

Methods

Participants

Written informed consent, approved by the institutional review board, was obtained from 45 physically active individuals aged 18 to 25 years who participated in this study. Participants were classified based on the type and frequency of the physical activity performed, as described in Table 1. The activity classification was developed by the authors and thought to be a good representation of a “recreationally active” individual. To be classified into one of the groups, participants were required to engage in 1 category of physical activity (either uniplanar or multiplanar) at least 3 times more per week than the other category. If the participant’s activity participation did not meet this requirement, he or she was excluded from the study. Multiplanar movement activities were defined as those that involved either cutting or jumping. Uniplanar movement activities were defined as those in which participants perform primarily sagittal plane movements (Table 1).

Participant Activity Classification

Table 1: Participant Activity Classification

The authors determined that some activities did not sufficiently qualify as uniplanar or multiplanar; therefore, participants who participated in these activities (Table 1) were excluded. Participants were also excluded if they reported any of the following: history of orthopedic injury to the trunk or lower extremity in the previous 6 months; history of back, hip, knee, ankle, or foot surgery; or any neurological condition or medication that impaired balance.

Power analysis using previously recorded data in our laboratory revealed that 15 participants were needed in each of the 3 groups to achieve a statistical power of β = .8 to detect differences in knee abduction during landing. Female participants were divided into 2 groups: multi-planar activity female group (n = 15, age = 20±2 years, height = 167.21±7.53 cm, mass = 65.61±6.59 kg) and uniplanar activity female group (n = 15, age = 22±2 years, height = 167.25±7.07 cm, mass = 62.72±7.03 kg). All male participants were recruited for the multiplanar activity male group (n = 15, age = 20±2 years, height = 180.66±6.75 cm, mass = 77.30±7.94 kg).

Instrumentation

A 3-dimensional motion analysis system (Flock of Birds; Ascension Technology Corp, Burlington, Vermont) interfaced with Motion Monitor software (Innovative Sports Training Inc, Chicago, Illinois) was used to record the 3-dimensional kinematics of the hip and knee during the landing task, at a sampling frequency of 100 Hz. Knee and hip joint angles were measured from the time of initial ground contact until peak knee flexion occurred. Each participant wore shoes containing insoles equipped with foot switches with sensors in the heel, midfoot, and toe (Datapac 2K2; RUN Technologies, Mission Viejo, California) to accommodate all types of landings. Each sensor used a unique voltage so that a change from 0 indicated each participants’ initial ground contact. The foot switch output was sampled at 2000 Hz and synchronized with the kinematic data.

Peak isometric hip abduction and external rotation strength was assessed with the Primus RS dynamometer (BTE Technologies, Hanover, Maryland). The Primus RS dynamometer was also used to induce hip abductor and external rotator muscular fatigue through an isometric fatigue protocol.

Procedures

Prior to the beginning of testing, data concerning the participants’ height, mass, age, and activity level were collected. Participants performed a 5-minute submaximal warm-up on a cycle ergometer. Participants then determined their preferred limb by self-selecting a leg to land on at least twice during 3 trials of a double-leg forward jump with a single-leg landing. Testing consisted of the following procedures: prefatigue landing task, isometric strength testing, isometric fatigue protocol, and postfatigue landing task. All testing was performed with the preferred leg.

Three electromagnetic sensors were attached directly to the skin of the preferred limb using adhesive tape. Sensors were attached to the proximal medial tibial border, the lateral thigh, and the sacral base.3,21 A local reference frame was used to calculate joint centers. Participants were required to wear standardized footwear (Air Max Challenge; Nike, Inc, Beaverton, Oregon).

The landing task consisted of a double-leg forward jump with a single-leg landing, similar to that described by Lephart et al3 and Jacobs et al.21 Participants began the task by placing both feet on the start line. An overhead target was hung at a height equivalent to 115% of each participant’s height (Figure 1). Participants were instructed to hit the overhead target with their forehead during each jump. A landing target was placed on the floor at a distance from the start line equal to 45% of the participant’s height. On landing, participants were instructed to stabilize themselves as quickly as possible and were allowed to use their arms. Participants were allowed ample time to become familiar with the landing task and were required to complete a minimum of 3 practice trials. Jumps were performed consecutively with no rest between repetitions.

Ending position for landing task, showing overhead (O) and landing targets (L).

Figure 1. Ending position for landing task, showing overhead (O) and landing targets (L).

As soon as a trial was recorded on the computer, participants were verbally encouraged to return to the line as quickly as possible and jump again. Three trials were collected for each participant. If the participant failed to correctly complete the jumping task (ie, landed on both feet, landed on the wrong foot, or failed to hit both targets), the trial was discarded and the participant was required to complete additional trials until a total of 3 successful trials had been recorded. Participants repeated the same landing task immediately following the isometric fatigue protocol.

For isometric strength testing, participants were placed in a side-lying position on a table, with the preferred leg superior. The trunk was in neutral alignment with the hips flexed to 45° and the knees flexed to 90°, as described by Nyland et al,25 to isolate the gluteus medius. Participants were allowed to support their heads using the contralateral hand and arm and could stabilize themselves on the table using the ipsilateral hand. Participants remained in this position for the duration of the strength testing and fatigue protocol.

The axis of rotation of the resistance arm of the dynamometer was aligned with the participants’ hip joint, and the pad at the end of the resistance arm was placed slightly above the lateral joint line of the participants’ knee. Participants then abducted and externally rotated the preferred limb against the pad. The foot of the preferred leg was not allowed to touch the other foot but was required to remain below the level of the knee25 to prevent hip internal rotation (Figure 2). To obtain maximum voluntary isometric contraction (MVIC) strength, participants performed 3 maximal hip abduction/external rotation contractions against resistance with verbal encouragement. Contractions lasted 5 seconds each, with a 30-second rest between contractions. Peak isometric torque for each MVIC trial was calculated in Newton-meters and normalized to body mass. After the isometric strength test, participants performed an isometric fatigue protocol.

Participant positioning for isometric strength test and isometric fatigue protocol.

Figure 2. Participant positioning for isometric strength test and isometric fatigue protocol.

For the fatigue protocol, participants maintained an isometric hip abduction/external rotation contraction for as long as possible. The isometric contraction was maintained at a load equal to 50% of mean MVIC. The protocol was terminated when the participant could no longer maintain the load for 3 consecutive seconds and was considered to have reached the desired level of muscular fatigue. Participants were provided with verbal encouragement to ensure that each participant gave maximal effort. At the completion of the fatigue protocol, participants immediately performed the postfatigue landing task, which was identical to the prefatigue landing task. The time between the completion of the fatigue protocol and the beginning of the postfatigue landing task was, on average, less than 20 seconds, with the entire postfatigue landing task lasting less than 2 minutes.

Data Reduction

All kinematic data were analyzed and processed using Datapac 2K2. A fourth-order zero-lag low-pass Butterworth filter with a cutoff frequency of 9 Hz was used to filter kinematic data. The dependent kinematic variables were excursions for the following motions: hip adduction, hip internal rotation, and knee abduction (degrees). Excursion was defined as the difference between the maximum angle obtained prior to maximum knee flexion and the angle at ground contact (excursion = maximum angle – angle at contact). Initial ground contact was identified by the activation of any of these 3 sensors within the foot switch. The kinematic analysis was modeled after previously reported methods for assessing single-leg landings.3,17,21

Statistical Analysis

For each condition, the average of 3 landing trials was used for statistical analysis. Three repeated measures analyses of variance (ANOVAs), with one between factor (group) and one within factor (fatigue), were used to compare means for each of the dependent variables for excursion. There were 3 levels for group: multiplanar activity female, uniplanar activity female, and multiplanar activity male. There were 2 levels for fatigue: pre-fatigue and postfatigue. For the strength comparison, a one-way ANOVA was used to compare normalized peak torque between groups. Significance was set a priori at P < .05. If a main effect was present, a Holmes Modified Bonferroni post hoc test was performed to correct for multiple comparisons and determine where significant differences existed.26 Statistical analyses were conducted using PASW Statistics version 18.0 software (PASW Statistics, Chicago, Illinois).

Results

There was a main effect for fatigue for hip adduction excursion (Table 2). Across groups, participants had a significantly greater excursion for hip adduction in the postfatigue condition (14.0°±4.8°) than in the prefatigue condition (10.6°±5.3°, P < .001). There was no significant difference in hip internal rotation excursion (P = .070) or knee abduction excursion (P = .454) between conditions.

Hip and Knee Joint Excursions by Condition

Table 2: Hip and Knee Joint Excursions by Condition

There was no significant group-by-condition interaction for hip adduction excursion (P = .673), hip internal rotation excursion (P = .490), or knee abduction excursion (P = .817). There was a main effect for group (Table 3). Post hoc testing revealed that the multiplanar activity male group had a significantly lower knee abduction excursion (4.4°±4.2°) than both the multiplanar (8.8°±4.8°, P = .018) and the uniplanar (12.5°±4.8°, P < .001) female groups. In addition, the 2 female groups demonstrated a difference in knee abduction excursion. The uniplanar activity female group had significantly greater knee abduction excursion than the multiplanar activity female group (P = .031) (Table 3). No main effect was determined for group for either hip adduction excursion (P = .163) or hip internal rotation excursion (P = .503).

Kinematic and Strength Data by Group

Table 3: Kinematic and Strength Data by Group

There was a group main effect for strength (Table 3). Peak hip abduction and external rotation torque of the uniplanar (1.00±0.25 N·m/kg) and the multiplanar (1.02±0.21 N·m/kg) activity female groups were significantly less than the peak torque of the multiplanar activity male group (1.24±0.26 N·m/kg, P = .024 and .036, respectively). There was no significant difference in normalized strength between the uniplanar and the multiplanar activity female groups (P = .757).

Discussion

Effect of Fatigue on Landing

We determined the effect of hip abductor and external rotator fatigue on 3-dimensional hip and knee kinematics during a jump landing. Our first hypothesis was partially supported by our results. A significant increase in hip adduction following the fatigue protocol was seen; however, this was the only dependent variable affected by our fatigue intervention. Jacobs et al21 reported an increase in hip adduction following a submaximal bout of isometric hip abduction exercise. Using the same jump landing protocol as Jacobs et al,21 we also demonstrated differences in hip adduction following fatigue.

Our protocol was successful at inducing muscular fatigue, as evidenced by the difference in hip adduction excursion across conditions. Despite this change in hip kinematics, knee abduction kinematics were not affected. This finding is also consistent with studies that reported no difference in knee valgus angle (knee abduction) immediately following a hip abductor fatigue protocol.13,21 This could mean several things. It is possible that our fatigue protocol did not fatigue the hip muscles to the extent that the position of the knee was adversely affected. It is also possible that other muscles, such as the hamstrings and quadriceps, or distal structures, such as the foot and ankle, may have been more responsible for controlling knee motion or may have compensated for any potential effect of the hip fatigue. In the absence of electromyography data, it is unknown whether muscle activation patterns may have changed in response to the hip abductor/external rotator fatigue.

Lateral trunk displacement has been observed to occur during ACL injury in women,27 and decreased neuromuscular control of the trunk has been identified as a predictor of knee injury.28 Given this description of the involvement of the trunk in landing stabilization, it is also possible that participants used compensatory trunk movements to stabilize their center of gravity during landing and that these movements were enough to maintain knee stability in the frontal plane.

In sport participation, central fatigue may be greater than that experienced in our protocol. We chose a moderate peripheral fatiguing protocol because we and our institutional review board were concerned with limiting the potential for serious injury. Our results demonstrate that localized hip abductor and external rotator fatigue can influence hip biomechanics during landing, resulting in an increase in hip adduction; however, the local fatigue did not influence knee biomechanics to the extent we had anticipated.

Effect of Activity Type on Landing

Although there is consistent evidence that women land in more knee abduction when compared with men,7,14–16,19,21–24 none of the aforementioned participants were classified by the activity type. Several studies7,15,16,19,20 included athletes who would be classified, according to our definition, as participating in multi-planar activities. The remainder of the studies did not differentiate among participants on the basis of the type of activity they performed.14,22,23 In fact, some of these studies’ inclusion criteria did not require the participant to even be physically active.21,24 In our study, a significant difference was observed in knee abduction between women who participate in different activity types.

Our second hypothesis was not supported by our results. There was no interaction between the two independent variables—activity type and fatigue. However, a main effect for activity type was seen; the uniplanar activity female group had a larger knee abduction excursion than did the multiplanar activity female group. Participants who performed jumping and cutting activities less frequently (uniplanar activity female group) landed in a position that has been associated with greater risk of ACL injury,29 whereas those who performed these maneuvers more frequently (multiplanar activity female group) landed in a safer, more neutral position.

This is the first study to examine differences in landing mechanics between athletes performing uniplanar activities and athletes performing multiplanar activities. Neuromuscular control training programs provide athletes with additional repetitions of multiplanar activity movement patterns, and some have been shown to help athletes decrease knee abduction angle during landing.30 We recommend future research examine whether postintervention changes in biomechanics are a result of specific training and education or simply a dosage response to increased participation in dynamic, multiplanar activities.

We also examined the strength differences between the activity groups. A significant difference in normalized peak torque between the uniplanar activity female group and the multiplanar activity male group and between the multiplanar activity female group and the multiplanar activity male group was observed (Table 3). In both cases, the multiplanar activity male group demonstrated a larger hip abduction/external rotation peak torque than the female groups. Similarly, Jacobs et al21 reported that men demonstrated significantly greater peak hip abduction torque than women when tested in a side-lying position, but reported no difference in peak torque when tested in a standing position.17

In the current study, differences were observed in knee abduction excursion between the uniplanar and multiplanar activity female groups, but no differences in strength were observed between these 2 groups (uniplanar activity female group, 1.00±0.35 N·m/kg; multiplanar activity female group, 1.02±0.21 N·m/kg). Therefore, the observed differences in kinematics do not appear to be due to hip abductor/external rotator weakness. This is further supported by the lack of a significant difference in knee abduction excursion from prefatigue to postfatigue, when the hip abductors/external rotators are presumably in a weakened state. The results of the current study suggest that hip abductor and external rotator strength may not be as influential on knee landing kinematics as previously thought. However, this relationship warrants further investigation through prospective studies examining the effect of hip strengthening protocols on hip and knee landing mechanics.

Effect of Gender on Landing

Our third hypothesis was that both female groups would exhibit landing kinematics different than those of the male group. This hypothesis was supported by our results. Both the multiplanar and the uniplanar activity female groups had significantly greater knee abduction excursion than the multiplanar activity male group. Our results are similar to those of others who reported differences in knee abduction angle between men and women.7,14–16,19,21–24 We observed that regardless of activity type, women landed with more knee abduction than men. Increased knee abduction angle has been demonstrated to be a predictor of high knee abduction moment during jump landing,31 which has been demonstrated to be a predictor of ACL injury.8

Limitations

There are several limitations to our study. We did not place a sensor on the thorax and, consequently, were unable to monitor trunk rotation and side bending, as well as any rotation that may have occurred at the pelvis. Likewise, the addition of electromyography would have provided additional information regarding activation pattern and fatigue level of the musculature studied. We did not include a uniplanar activity male group in our initial study design because we felt that we would find the greatest differences between the 3 groups we chose and for ease of participant recruitment. Finally, we did not account for the number of years of participation in a particular activity when we categorized participants. We based our classification on participants’ current activity and did not account for previous activities that participants that may have participated in at an earlier time in their lives.

Conclusion

Fatigue of the hip musculature resulted in changes in kinematics during landing. Following fatigue, participants experienced more hip adduction during landing. Regardless of fatigue, women who participated in either multiplanar or uniplanar activities landed in more knee abduction than did men. In addition, women who participated in uniplanar activities landed in more knee abduction than those who participated in multiplanar activities. It is our recommendation that future research include activity type in participant inclusion criteria to decrease variability and obtain more accurate results. The results of the current study demonstrate that landing kinematics are influenced by activity participation, as well as gender.

Implications for Clinical Practice

Our results support the theory that athletes who frequently participate in multiplanar movements land in a position of less knee abduction. Therefore, we recommend that athletes performing uniplanar activities or multi-sport athletes, such as those found in many high school settings, should be encouraged to cross-train by incorporating additional landing and cutting maneuvers into their workouts when they are engaged in sports where these movements are not predominate. In addition, the findings of the current study emphasize the importance of classifying participants’ activity type when designing research studies and analyzing results. Our study demonstrates that differences exist between female athletes in executing a landing task depending on the type of activity in which they most commonly participate. Therefore, we recommend that activity type be considered as a covariate or analyzed separately when performing statistical analyses of the biomechanical assessment of a landing task.

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Participant Activity Classification

MULTIPLANARUNIPLANAREXCLUDED
Step aerobicsRunningWeightlifting
SoccerCross countryYoga
VolleyballSwimmingPilates
BasketballBikingBaseball
Flag footballEllipticalSoftball
Field hockeyStairmaster
TennisArc trainer
RacquetballSpinning
Rugby

Hip and Knee Joint Excursions by Condition

MOTIONJOINT ANGLE (°)
P
PREFATIGUEPOSTFATIGUE
Hip adduction
  Contact−7.49±5.4−6.68±6.0<.001a
  Excursion10.6±5.314.0±4.8
Hip internal rotation
  Contact−12.14±6.9−9.10±7.4.07
  Excursion7.59±4.68.50±4.6
Knee abduction
  Contact−4.01±5.8−4.11±5.8.454
  Excursion8.42±5.88.73±5.5

Kinematic and Strength Data by Group

GROUPJOINT ANGLE (°)
HIP ABDUCTOR/EXTERNAL ROTATOR (MVIC [N·M/KG])
HIP ADDUCTIONHIP INTERNAL ROTATIONKNEE ABDUCTION
Uniplanar activity, female13.45±6.08.86±5.312.5±4.8a1.00±0.25b
Multiplanar activity, female13.0±4.58.24±4.08.8±4.81.02±0.21c
Multiplanar activity, male10.37±5.17.02±4.44.4 ±4.2b,c1.24±.026
Authors

Ms Fazio is from the University of Colorado, Boulder, Colorado; Dr Howard, Dr Mattacola, and Dr Uhl are from the University of Kentucky, Lexington, Kentucky; Dr Jacobs is from ArthroMetrix LLC, and ERMI Medical Devices, LLC, Atlanta, Georgia.

This study was funded by the National Athletic Trainers Association Research and Education Foundation (#505MGP004), Dallas, Texas. The authors thank Robert Shapiro, PhD, for his contribution to the conception and design of this study.

The authors have no financial or proprietary interest in the materials presented herein.

Address correspondence to Melisa A. Fazio, MS, ATC, University of Colorado, Dal Ward Building, 100 Stadium Drive, 368 UCB, Boulder, CO; e-mail: Melisa.fazio@colorado.edu

10.3928/19425864-20111017-02

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