Journal of Applied Biomechanics, 2014, 30, 655-662 http://dx.doi.org/10.1123/JAB.2013-0331 © 2014 Human Kinetics, Inc.

An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH

The Effects of a Lateral In-flight Perturbation on Lower Extremity Biomechanics During Drop Landings Jae P. Yom,1 Kathy J. Simpson,2 Scott W. Arnett,3 and Cathleen N. Brown2 1University

of South Dakota; 2University of Georgia; 3Western Kentucky University

One potential ACL injury situation is due to contact with another person or object during the flight phase, thereby causing the person to land improperly. Conversely, athletes often have flight-phase collisions but do land safely. Therefore, to better understand ACL injury causation and methods by which people typically land safely, the purpose of this study was to determine the effects of an in-flight perturbation on the lower extremity biomechanics displayed by females during typical drop landings. Seventeen collegiate female recreational athletes performed baseline landings, followed by either unexpected laterally-directed perturbation or sham (nonperturbation) drop landings. We compared baseline and perturbation trials using paired-samples t tests (P < .05) and 95% confidence intervals for lower-extremity joint kinematics and kinetics and GRF. The results demonstrated that perturbation landings compared with baseline landings exhibited more extended joint positions of the lower extremity at initial contact; and, during landing, greater magnitudes for knee abduction and hip adduction displacements; peak magnitudes of vertical and medial GRF; and maximum moments of ankle extensors, knee extensors, and adductor and hip adductors. We conclude that a lateral in-flight perturbation leads to abnormal GRF and angular motions and joint moments of the lower extremity. Keywords: anterior cruciate ligament, joint kinematics, joint kinetics, landing, external impact force Injury to the anterior cruciate ligament (ACL) is both common and costly. For example, 250,000 ACL injuries are estimated to occur to one in 3000 people in the United States annually,1 and associated medical costs are approximately $17,000 per injury.2,3 Thus, the annual cost of ACL injury rehabilitation is more than two billion dollars. Furthermore, the ACL-injured population has a higher risk of serious, long-term consequences compared with an uninjured population.4 More than 70% of ACL injuries are associated with a meniscus rupture or tear of the collateral ligament.4,5 In addition, even with satisfactory reconstruction of the ACL, injured individuals have a higher risk of developing osteoarthritis compared with an equivalent, uninjured population.4,6,7 We know much about ACL injury etiology8–11 and the mechanics of one of the most common scenarios during which ACL injuries occur, which is the landing phase after being airborne.11–14 However, one possible injury scenario that is poorly understood is that of the cascading biomechanical effects on a performer who has come into contact with another player during the aerial phase. At contact, the performer must maintain an appropriate position to attenuate high impact forces while minimizing the strains and stresses to the ACL. However, if the athlete is perturbed unexpectedly in the air by a teammate or opponent in a lateral direction, Newtonian physics supports that the athlete’s flight phase movements are altered. Hence, compared with a typical landing at the instant of touchdown, the athlete’s body may not be in optimal alignment for absorbing the ensuing high impact forces and attenuating the Jae P. Yom is with the Biomechanics Laboratory, University of South Dakota, Vermillion, SD. Kathy J. Simpson and Cathleen N. Brown are with the Biomechanics Laboratory, University of Georgia, Athens, GA. Scott W. Arnett is with the Biomechanics Laboratory, Western Kentucky University, Bowling Green, KY. Address author correspondence to Jae P. Yom at [email protected].

high body inertia. We anticipated, therefore, that an abnormal landing would lead to abnormal knee motions that could, subsequently, disrupt stability and result in abnormal inertial and ground reaction forces (GRF) as well as compensatory lower extremity joint moments. Consequently, these reactions could indirectly result in excessive strain and stress to the knee joint during the landing phase. Moreover, the performer may be unable to use typical landing strategies, that is, lower extremity flexion combined with eccentric extensor muscle torques that act to reduce the body’s kinetic energy safely via negative angular muscle work.15 In addition, upon landing after a lateral perturbation, initially there may be a smaller base of support, reducing stability. Hence, either excessive abduction or adduction joint motion could occur to control frontal plane motions of the trunk and lower extremity segments or greater frontal plane moments must be generated to counteract or prevent excessive motions. Moreover, if there is abnormal mediolateral joint alignment at initial ground contact (IC), greater moments due to joint reaction forces may occur, as their moment arms would be longer. In addition, with less ability to attenuate vertical GRF (GRFvertical) and body inertial forces, these forces would also increase joint reaction force moments. Hence, it is imperative to determine the effects of mediolateral perturbation on landing biomechanics of women to better understand ACL injury causation. We wish to understand these effects among female athletes because, relative to sex, female athletes’ ACL injury rate is two to eight times higher than male counterparts of comparable athletic skill.16,17 Furthermore, several researchers have reported sex differences of frontal plane lower extremity kinematics during landing movements.18,19 Therefore, the overall purpose of the study was to determine the biomechanical effects of lateral flight-phase perturbations applied to female athletes during drop landings. For kinematics, we hypothesized that compared with the baseline landings, the perturbation landings would display decreased lower extremity joint 655

656  Yom et al.

flexion displacement. Furthermore, increased ankle inversion, tibial abduction, and hip adduction displacement would be shown between IC and the end of the landing phase. For the GRFs, magnitudes of the peak GRFvertical and medial GRF (GRFmedial) were expected to increase during perturbation landings. In addition, shorter time to peak GRFvertical was expected during perturbation landing due to decreased lower extremity joint flexion displacement. In addition, magnitudes of knee adductor and extensor net muscle moments of the lower extremity were expected to increase during perturbation landings.

Methods

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Participants Seventeen female participants (mean ± SD: age = 21.1 ± 1.3 y; mass = 61.5 ± 9.9 kg; height = 166.6 ± 6.0 cm) were recruited from intramural teams, sport club teams, and physical activity classes. The university’s institutional review board approved all forms and research protocols. Written informed consent was given by each participant before data collection. Eligibility to participate was assessed using the participant’s answers to a laboratory health status, medical history, and physical activity questionnaire. Participants had to be healthy, without any current or chronic injury or condition that could potentially affect the participant’s performance or safety.

load cell (MLP-200; Transducer Techniques, Temecula, CA) at the other end. The load cell was attached to a vest harness worn by the participant at the other end. The cable and load cell was attached to the harness at the location of the acromioclavicular joint on the same side of the body as the preferred landing leg (the side ipsilateral to the dominant leg). The preferred landing leg was defined as the leg that the participant used to kick a ball.21 The electrical signals from the load cell were later used to confirm the magnitude of the perturbation impulse. To initiate the perturbation, as the participant let go of the drop bar and as the grip force dropped below 13.2 N, the main device was triggered. Pressurized air from an air compressor caused one air piston to pull out the restraint blocking a second piston. Subsequently, when the second air piston was allowed to move, the perturbation cable was pulled, thereby transmitting a perturbation force (pressure setting on perturber = 1.15 × body mass) to the participant in a lateral direction in a horizontal plane during the flight phase.

Drop Test Procedures

Two Bertec force platforms (Model: 4060-NC; Bertec Corp., USA) were used to collect GRF (1200 Hz) from three directions: GRFanteroposterior, GRFmediolateral, and GRFvertical. Seven Vicon MX40 cameras and Workstation software (Vicon Inc., Englewood, CA) were used to capture (240 Hz) the spatial locations of 40 retro-reflective markers placed on each participant’s lower extremities and her own shoes using the marker set guidelines from the C-Motion (C-Motion, Inc., Rockville, MD) for lower extremity. A specially-made machine (the ‘perturber’; Figure 1) was used to generate the perturbation during the flight phase of drop landings.20 The perturber consisted of a trigger sensor system that included a strain gauge placed on the drop bar, a perturbation cable and load cell, and the main device that created the impulse applied to the participant (the perturbation). The perturbation cable, a wire cable, was attached to the main device at one end and a tensile

Participants began each trial hanging from the drop bar with both hands (height from the midpoint of the lateral malleolus to the ground = 0.55 m; Figure 2). On cue, the participant released her hands from the drop bar and landed with one foot on each force platform. The participant kept the arms at the initial shoulder position during the entire movement. A trial was determined acceptable if each foot landed entirely on the correct force platform and the participant remained stable upon landing for 2 seconds. The participant performed two blocks of testing. The first block of testing (baseline) consisted of three acceptable trials of natural drop landings with no expectation of a perturbation. Before the second block of testing, the perturbation cable was attached. The second block of testing consisted of three acceptable trials of the perturbation condition and three sham (nonperturbation) trials, which were performed in a quasi-random order. No more than two trials of perturbation condition (or sham) were performed in subsequent order. This test order and the use of sham trials were done to reduce feedforward control, anticipatory effects, and the development of adaptive strategies.22 The participant had approximately 15–20 seconds of rest between trials to minimize neuromuscular fatigue. Any unacceptable trial was repeated after completing all other trials. An unacceptable trial was defined as any part of the participant’s foot landing outside

Figure 1 — Picture of ‘perturber.’

Figure 2 — The experimental set-up to perturbation landing.

Instrumentation

In-flight Perturbation and Drop Landing   657

of the correct force platform or the participant qualitatively appearing (or self-reporting) to have difficulty becoming stable during the landing phase.

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Data Reduction Biomechanics of the dominant limb were analyzed for the baseline and perturbation trials of the landing phase. The nonperturbation trials were not included, as their major purpose was to reduce anticipation of the perturbation during the second block of testing. The landing phase began at the instant of initial ground contact (IC; GRFvertical > 5 N) and ended at the first instant when maximum knee flexion angle occurred. GRF data reduction was performed using Visual 3D software (C-Motion Inc., Germantown, MD). The GRF raw data were filtered using a 15 Hz low-pass, fourth-order Butterworth filter. Peak GRF magnitudes for GRFvertical, GRFanterior, and GRFmedial were scaled to body mass. In addition, time to peak GRFvertical was generated. Reconstruction of the three-dimensional coordinates of the reflective markers were generated via a proprietary algorithm (Workstation, Vicon Inc., Englewood, CA) and smoothed using a 15 Hz low-pass, fourth-order Butterworth filter. All other calculations were performed using Visual 3D software. Cardan angles were used to generate subsequent joint coordinate systems for the ankle, knee, and hip joints of the landing legs to determine clinical joint angles.23 Angular kinematic variables of all three axes of the ankle, knee, and hip joints consisted of joint angles at IC, the maximum joint angle, and joint angular displacement displayed during landing. Three-dimensional joint moments of the lower extremity were calculated using standard inverse dynamics (Visual 3D) and scaled to body mass and height. The maximum joint moment magnitudes that were displayed by all participants were analyzed.

Statistical Analysis Paired-samples t tests were used to detect differences between the baseline and perturbation landing conditions (P < .05). Confidence intervals (CI) at 95% level were also generated to assess behavioral significance (SPSS v.20; IBM, Inc., Chicago, IL).

Results For lower extremity frontal plane joint kinematics, differences were noted between perturbation and baseline landings. Participants exhibited 1.5° less ankle inversion at IC; however, no other ankle joint kinematic differences were observed (Figure 3 and Table 1). At the knee joint, perturbation trials resulted in an increase of 1.6° and 2.4° for the IC abduction angle and maximum abduction

Figure 3 — Means and standard deviation of initial contact and maximum angles and maximum angular displacement. Differences between baseline (BASE) and perturbation (PERT) landings (white bars = initial joint angle; black bars = maximum joint angle). *Significant difference at initial ground contact (IC) (P < .05). §Significant difference at maximum angles (P < .05). ‡Significant difference at maximum angular displacement (P < .05). Jt = joint.

Table 1  Means ± SD and the lower and upper bounds of 95% CI for differences between perturbation and baseline landings of initial contact and maximum joint angles and joint displacement variablesa Joint Axis Adduction/Abduction (Inversion/Everson) Joint Ankle

Knee

Hip

Variable

Mean ± SD

95% CI LB—UB

p Value

Flexion/Extension Mean ± SD

95% CI LB—UB

p Value

IC

–1.5 ± 1.7

–2.3 to –0.6

< .001

3.0 ± 3.3

1.3 to 4.3

.002

Max.

0.0 ± 2.4

–1.2 to 1.3

.984

–0.9 ± 2.6

–2.2 to 0.5

.191

Disp.

–1.5 ± 2.3

–2.7 to –0.3

.002

2.2 ± 4.8

–0.3 to 4.7

.077

IC

1.6 ± 1.0

1.1 to 2.1

< .001

–2.1 ± 3.0

–3.7 to –0.6

.010

Max.

2.4 ± 2.8

0.9 to 3.8

< .001

–3.1 ± 5.1

–5.7 to –0.4

.030

Disp.

0.8 ± 2.5

–0.5 to 2.1

.206

–0.9 ± 5.1

–3.6 to 1.7

.479

IC

4.1 ± 2.3

2.9 to 5.2

< .001

–0.9 ± 2.0

–1.9 to 0.1

.080

Max.

–0.2 ± 2.4

–1.4 to 1.1

.778

–1.8 ± 3.2

–3.4 to –0.1

.037

Disp.

4.2 ± 2.1

3.2 to 5.3

< .001

–0.9 ± 3.2

–2.5 to 0.7

.271

Note. CI = confidence interval; LB = lower bounds; UB = upper bounds; IC = initial contact; Max. = maximum joint angles. Bold p value = a significant t test comparison (p < .05). a A positive or negative score indicates that the value of the perturbation condition was greater or lesser, respectively, than the baseline value.

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658  Yom et al.

angle, respectively. Hence, the CI did not support significance for knee abduction displacement. However, participants produced a 4.2° increase in hip adduction displacement during perturbation compared with baseline landings. Sagittal plane joint kinematics at the ankle, knee, and hip joints also were different between the perturbation and baseline conditions. At IC, compared with baseline, perturbation landings resulted in a 3.0° greater plantar flexed ankle and 2.1° less flexed knee joint position. In addition, compared with baseline, during the perturbation condition participants exhibited 3.1° and 1.8° less maximum knee and hip flexion angles, respectively (Figure 3 and Table 1). Therefore, there were no significant differences for sagittal plane displacements about any joint. As hypothesized, greater peak GRFs were exhibited during the perturbation condition, compared with baseline (Figure 4, Table 2). The peak GRFvertical, GRFmedial, and GRFanterior acting on participants during the perturbation condition increased by 3.8 N⋅kg–1, 0.9 N⋅kg–1 and 0.7 N⋅kg–1, respectively. However, no significant difference for time to peak GRFvertical between perturbation and baseline conditions existed. Differences between perturbation and baseline landings were also noted for sagittal plane knee and ankle joint kinetics. Compared with baseline, perturbation landings elicited a 0.11 N⋅m⋅(kg⋅m)–1 greater peak knee extensor moment and a 0.18 N⋅m⋅(kg⋅m)–1 greater peak ankle plantar flexor moment (Figure 5 and Table 3). However, the peak hip extensor moment was not significantly different between conditions. Furthermore, there were significant differences in the frontal plane (Table 4). When comparing the frontal plane joint kinetics, perturbation showed 0.01 N⋅m⋅(kg⋅m)–1 greater ankle inversion moment compared with baseline. At the knee joint, during perturba-

tion compared with baseline landings, the first and maximum adduction joint moments were 0.04 N⋅m⋅(kg⋅m)–1 and 0.1 N⋅m⋅(kg⋅m)–1 greater, respectively. However, the effect of perturbation was not meaningful, as shown by the CI. For hip joint moments, only the second hip adduction moment was significant, displaying a 0.08 N⋅m⋅(kg⋅m)–1 greater moment during perturbation landings.

The aim of this study was to determine the lower extremity biomechanical responses exhibited by females during landings after incurring a laterally-directed, in-flight perturbation. The results of this study mostly support the hypotheses and some of the anticipated justifications. We had surmised that, due to the perturbation, the body would gain angular and linear momenta in the frontal plane during the flight phase. In turn, we hypothesized that these momenta would lead to different landing angles and increased angular displacements, joint moments in the frontal plane, and GRFmedial as compared with baseline landings. In addition, we also anticipated decreased lower extremity joint flexion displacements and, therefore, increased lower extremity extensor moments, increased GRFvertical, and decreased time to peak GRFvertical compared with baseline landings. For the IC angles of the lower extremity, it was anticipated that perturbation compared with baseline would display a greater ankle inversion, knee abduction, and hip adduction position. Support for our predictions for the frontal plane IC angle was mixed. The knee joint was in a greater abducted position as expected; however, the hip joint also was in a greater abducted rather than adducted position. Moreover, the ankle landed in a more neutral rather than a more inverted position.

Figure 4 — Means and standard deviation of ground reaction force (GRF) variables. Difference between baseline and perturbation landings; * = significant difference (P < .05).

Figure 5 — Means and standard deviation of the sagittal plane joint moments. Differences between baseline (black bars) and perturbation (white bars) landings.*Significant difference at IC (P < .05). Jt = joint.

Table 2  Mean ± SD and 95% confidence intervals (CI) for peak GRF (N⋅kg–1) and time to peak vertical GRF (ms)a

Table 3  Mean ± SD and the lower and upper bounds of 95% confidence interval for peak extensor joint moments (N⋅m⋅[kg⋅m]–1)a

Discussion

Variable

Mean ± SD

95% CI

p Value

Joint

Mean ± SD

95% CI LB—UB

p Value

Peak GRFvertical

3.80 ± 4.26

1.61–5.99

.002

Hip

0.07 ± 0.27

-0.06–0.21

.276

Peak GRFmedial

0.90 ± 0.90

0.42–1.37

.001

Knee

0.14 ± 0.2

0.06–0.22

.002

Peak GRFanterior

0.70 ± 1.20

0.08–1.31

.030

Ankle

0.19 ± 0.09

0.14–0.24

< .001

Time GRFvertical

0.65 ± 8.80

-3.87–5.18

.763

Note. GRF = ground reaction force. Bold p value = a significant t test (p < .05). a A positive or negative score indicates that the value of the perturbation condition was greater or lesser, respectively, than the baseline value.

Note. LB = lower bounds; UB = upper bounds; CI = confidence interval. Bold p value = a significant t test comparison (p < .05). a A positive or negative score indicates that the value of the perturbation condition was greater or lesser, respectively, than the baseline value.

In-flight Perturbation and Drop Landing   659

Table 4  Mean, SD, and the lower and upper bounds of 95% confidence interval of difference scores for peak frontal plane joint moments (N⋅m⋅[kg⋅m]–1)a Joint

Joint Moment

Mean ± SD

95% CI LB—UB

p Value

Hip

Adductor

0.03 ± 0.14

-0.04–0.10

.346

Abductor

0.02 ± 0.17

-0.06–0.11

.614

Adductor

0.08 ± 0.01

0.00–0.16

.045

Knee

Ankle

Adductor

0.04 ± 0.04

0.02–0.06

.001

Abductor

–0.05 ± 0.10

-0.11–0.00

.068

Adductor

0.10 ± 0.08

0.02–0.06

< .001

Inversion moment

0.01 ± 0.01

0.00–0.02

.008

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Note. LB = lower bounds; UB = upper bounds; CI = confidence interval. Bold p value = a significant t test comparison (p < .05). a A positive or negative score indicates that the value of the perturbation condition was greater or lesser, respectively, than the baseline value.

One possible explanation is that the in-flight perturbation pulled the upper trunk laterally without having affected the lower extremity due to inertial lag of the lower extremity. Consequently, the more lateral position of the upper trunk would have created the increased hip abduction angle, not the upper leg positioning. For the frontal plane joint displacements, our hypotheses were also partially confirmed. As expected, there was greater displacement about the hip joint due to the perturbation, but not about the knee and ankle joints. Interestingly, the hip joint displacement difference was due to motion that was in the opposite direction during the perturbation (adduction) relative to the baseline condition (abduction). One possible explanation for this finding relates to the lateral trunk flexion. If the trunk laterally flexed due to the perturbation during the flight phase, then the participant would have to regain an upright position during the landing phase. At the knee joint, there are potentially four reasons for the lack of significant difference for abduction and adduction displacement. First, during perturbation landings the knee joint exhibited greater abducted positions from IC throughout the landing phase, but the displacement did not change. Second, as greater peak knee adductor moments were generated during the perturbation landing phase, it is suggested that the adductor muscles and other involved structures likely controlled the amount of abduction motion during the landing phase. Third, the perturbation did not have a consistent effect among all participants. Qualitatively, during perturbation landings, five participants demonstrated abduction displacement while four participants showed adduction displacement. Fourth, for some participants, there may have been no perturbation effect, as eight participants displayed a difference of less than 2° between the landing conditions. Behaviorally, increased knee joint abduction angles and greater hip adduction displacement exhibited during the perturbation versus the baseline condition suggest, indirectly, that a lateral perturbation could potentially increase the risk of an ACL injury to the leg of interest (ie, the leg ipsilateral to the side of the perturbation). Increased knee abduction at touchdown and throughout the landing phase is considered to be an ACL injury risk factor, as knee abduction may increase the strain and place greater stress on parts of the ACL.18,24–26 Additionally, Hewett et al27,28 confirmed this relationship between frontal plane joint movements and ACL injury. For most of the females incurring an ACL injury, greater lateral trunk flexion and knee abduction angles were observed compared with the

ACL-injured males. It is not known, however, if these associations were part of the injury causation mechanism. For the sagittal plane joint kinematics, we expected that joint displacements would be less during perturbation as opposed to baseline landings due to the lesser amount of flexion that would occur during the landing, not due to IC angles. Decreased sagittal displacements were surmised as a strategy to prevent abnormal frontal plane joint alignments and motions from occurring during landing after the perturbation. However, none of our predictions of the sagittal joint kinematics were supported. Two reasons may account for our lack of sagittal plane displacement differences. First is that the participant may subconsciously stiffen the joints in anticipation of the perturbation. Evidence exists that participants during perturbation landings exhibited more extended ankle, knee, and hip joint positions at initial contact. Second, for the perturbation condition, the participant did not change their technique during the rest of the landing to increase lower extremity flexion. The maximum knee flexion angle did not change; there was < 1° difference between landing conditions. Future comparisons of the sham (nonperturbation) condition and these conditions will allow us to test this notion of feedforward anticipatory effects. It should be noted that the extended knee landing position has potential behavioral consequences. A negative consequence is that an extended knee joint position at IC of landing movements is considered a risk factor for ACL injuries.29,30 This may be due to the anterior shear force that acts on the proximal tibia, as it is highest when the knee joint is in an extended landing position.31 However, a potential positive consequence of a more extended lower extremity at IC is that it can allow for greater flexion displacement during the rest of the landing phase, which is considered to be an effective part of a shock-attenuation strategy.32,33 Though the more extended IC angles appeared not to be associated with altered flexion displacements in this study. Our expectations that perturbation landings compared with baseline would elicit greater peak GRFvertical and GRFmedial were supported. Peak GRFmedial was increased by 9% during perturbation landings. One potential rationale was our original explanation. Increased lateral momentum of the body, including the leg of interest, would cause the need for greater opposing GRFmedial impulse to be applied to that leg. Moreover, the abnormal lower extremity alignments in the frontal plane during perturbation would

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660  Yom et al.

generate increased GRFmedial when the person pushed laterally against the ground to return the laterally-shifted COMbody and the lower extremity back to a more neutral position. In the vertical direction, for peak GRFvertical, perturbation elicited 39% greater force relative to body mass compared with baseline. Our original explanation was not supported. We anticipated less lower extremity flexion displacement during perturbation versus baseline landings that would then lead to increased peak GRFvertical. Decreased magnitudes of lower extremity flexion displacement during landings have been shown to increase vertical impact forces.32 However, the flexion displacements were not significantly less during perturbation landings in our study. Therefore, another possible explanation is that participants increased the lower extremity extensor joint moments that would indirectly result in pushing downward against the ground with greater force.34 Behaviorally, higher peak GRFvertical is considered a biomechanical risk factor for ACL injury, particularly for females.25,35 Using inverse dynamics data and ACL-related cadaver data to model ACL loading, Kernozek and Ragan36 reported that GRFvertical plays an important role in affecting the amount of force and strain placed on the ACL. For the joint moment outcomes, it was anticipated that increased lower extremity extensor moments during perturbation compared with baseline landings would be displayed. The results for the ankle and knee joints supported our suppositions. Compared with baseline, perturbation landings showed greater peak eccentric ankle plantar flexor and knee extensor moments during the landing phase. These moments increased during perturbation landings potentially to control the downward momentum without increasing lower extremity flexion. Our results of the extensor joint moments were similar to Arnett, who investigated the effects on landing biomechanics of a somersault axis in-flight perturbation applied in the anterior direction.20 It was, to our knowledge, the first study to investigate the effects of an in-flight perturbation on drop landing mechanics. He reported that the perturbation condition showed greater lower extremity extensor moments compared with the baseline condition. He surmised that participants used increased lower extremity moments, however, to reduce the somersault momentum. Although we cannot directly elucidate the effects of the lateral perturbation on ACL injury mechanisms, the increased knee extensor joint moment demonstrated during the perturbation condition could suggest increased ACL loading, based on a prior study.37 If the increase in knee extensor moment was due in part to a greater quadriceps muscle moment or reduced hamstrings and triceps surae moments, then greater anterior tibial shear force relative to the femur may be implied.37–39 Consequently, increased ACL loading and strain could be produced.31,40 It was anticipated that interindividual participant variability would be displayed for hip and knee abductor and adductor joint moment patterns,41,42 but not for the ankle inversion and eversion patterns. Therefore, it was surprising to find that among the lower extremity joints, the hip joint displayed the most consistent moment patterns among participants. All participants displayed hip of adductor-abductor-adductor-abductor moments from IC to the end of the landing phase. The individual participant variation observed in this study supports Hewett et al,25 finding that not all female athletes land similarly. They observed that females that later suffer an ACL injury may display 2.2 times greater knee abduction moment during a jump-landing task compared with their noninjured female counterparts. By limiting hip and knee sagittal plane motion, greater knee valgus motion and knee adductor joint

moments occur.43 This explanation is supported by our frontal and sagittal plane joint kinematic outcomes. We also had surmised that the knee adductor joint moment would increase during perturbation landings. Our results supported our hypothesis. Greater knee adductor moments (first and peak) and peak hip adductor (but not peak hip abductor) moments were displayed for perturbation landings compared with baseline. We assumed that the abnormal knee abduction alignment at IC due to the in-flight perturbation would require greater knee adductor joint moments during the landing phase when returning to neutral landing position. In summary, the lateral flight-phase perturbations created increased peak magnitudes of GRFvertical and GRFmedial. Furthermore, participants landed in a more extended position while the knee was abducted and hip abducted at IC. The IC and peak joint angles were more affected than joint displacements. Lack of displacement differences was due to differing participant responses to the perturbation or perhaps an attempt to minimize excessive abduction and adduction motions. In addition, knee extensor and adductor net muscle moments were increased due to the in-flight perturbation. Increased moments may have positive and negative consequences associated with ACL injury mechanisms. A limitation of the study is that the perturbation was generated in only one direction, which is not realistic in athletic contexts. Moreover, although sham trials were mixed in among perturbation trials to reduce perturbation anticipation, the participants knew the direction from which the perturbation would come. We conclude that lateral in-flight perturbations affect both frontal plane and sagittal plane lower-extremity biomechanics. Behaviorally, these outcomes indicated that female athletes typically adapt their mechanics to land safely after a semianticipated frontal plane perturbation, such as a push or a bump sideways by an opponent while in the air. The ability to generate sufficient, eccentric extensor and adductor moments about the lower extremity joints is of primary importance. Simultaneously, potential mechanisms for a soft-tissue injury during landing after a lateral perturbation have begun to be identified via our findings. A more extended, abducted knee and adducted hip at initial ground contact combined with increased GRFvertical, GRFmedial, and eccentric knee joint moments suggests that future study is needed to see if these outcomes are related to an increased injury risk. Acknowledgments This study was supported by the Mary Ella Lunday Soule Research Award from The University of Georgia. We thank Ashley Sherman and Michael Balaban for their assistance with data collection.

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