The Knee 22 (2015) 298–303

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The Knee

Sex differences in unilateral landing mechanics from absolute and relative heights Joshua T. Weinhandl ⁎, Bobbie S. Irmischer, Zachary A. Sievert Neuromechanics Lab, Department of Human Movement Sciences, Old Dominion University, Norfolk, VA 23529, United States

a r t i c l e

i n f o

Article history: Received 1 December 2014 Received in revised form 2 February 2015 Accepted 17 March 2015 Keywords: Knee Anterior cruciate ligament Kinematics Kinetics Energetics

a b s t r a c t Background: The prevalence of anterior cruciate ligament injuries in athletic populations and the sex disparity in injury rates are well documented. It is also recognized that landing from a jump is a common noncontact injury mechanism. Yet, most studies utilize absolute landing heights, and few have utilized landing heights equal to participants' maximal jumping ability. The purpose of this study was to examine unilateral landing mechanics from relative and absolute heights. Methods: Twenty-one female and twenty male participants completed a series of landings from absolute heights of 30, 40, and 50 cm, as well as a height equal to their maximum jumping ability. Right leg three-dimensional kinematics, kinetics, and energetics were calculated from initial contact to maximum knee flexion. Results: Females landed with greater peak posterior ground reaction force compared to males. Additionally, both female and male participants utilized the knee as the primary energy absorber, but females appear to emphasize greater ankle energy absorption compared to males. Females also displayed increased peak knee adduction moment, while males displayed decreased peak hip abduction moment as landing height increased. Conclusions: It appears that females and males respond to increasing landing heights differently. However, landings from 40 and 50 cm may have represented an unrealistic mechanical demand for females, and influence subsequent inferences regarding ACL injury risk. Therefore, we suggest that comparisons between studies utilizing different landing heights be made with caution, and participants jumping ability be taken into account whenever possible. Clinical relevance: The findings of this study offer novel insights with regard to landing height and lower extremity mechanics with the potential to inform anterior cruciate ligament injury intervention programs. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The prevalence of anterior cruciate ligament (ACL) injuries in athletic populations is well documented [1–3], and a majority of these injuries are reported to be caused by noncontact mechanisms [4]. These noncontact mechanisms typically include sudden deceleration and/or rapid direction changes such as landing from a jump, cutting, or pivoting motions [5]. Although ACL tears can occur during bilateral landings, unilateral landings are considered more dangerous due to a decreased base of support and increased demand on musculature of only one leg to absorb the impact [4]. Furthermore, recent epidemiological evidence indicates that females are more than twice as likely to have a first-time noncontact ACL injury compared to males [6]. This increased risk of injury, along with increased female participation in high school and collegiate sports, has led to a rapid rise in ACL injuries in female athletes [5] ⁎ Corresponding author at: Department of Human Movement Sciences, Old Dominion University, 1005 Student Recreation Center, Norfolk, VA 23529, United States. Tel.: +1 757 683 4754; fax: +1 757 683 4270. E-mail address: [email protected] (J.T. Weinhandl).

http://dx.doi.org/10.1016/j.knee.2015.03.012 0968-0160/© 2015 Elsevier B.V. All rights reserved.

and fueled many task- and sex-specific mechanistic investigations [7–20]. In general, females land with increased peak vertical [11] and posterior ground reaction forces (GRFs) compared to males [8]. Females also perform playing actions with decreased hip flexion, hip abduction, and knee flexion and knee abduction [7–10]. Furthermore, compared to males, females exhibit increased frontal plane hip and knee loading [7–10]. Finally, while males rely on the larger hip musculature to absorb energy, females absorb more energy at the knee and ankle [10–13]. These sex differences in landing kinematics, kinetics, and energetics have been attributed to decreased use of hip musculature to absorb the forces [11,12]. The effect of landing height (LH) on landing mechanics, and apparent injury risk, has also been well documented [21–25]. For example, lower extremity joint moments and work increase with increasing LH [22–25]. However, there is a divergence in knee joint kinematics between males and females as LH increases [21], suggesting that the relative demand of landing tasks may vary across individuals, and it may be beneficial to evaluate sex differences in landing mechanics from heights relative to maximum jumping ability.

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While several studies on lower extremity landing mechanics utilize absolute heights, few studies have examined those same variables while participants land from a height relative to jumping ability [10, 26] or equalized task demand [15]. When landing from a height relative to jumping ability, females exhibit difference in hip and knee kinematics [10]. Specifically, females exhibit decreased hip and knee flexion range of motion, as well as decreased hip abduction at initial contact compared to males [10]. These findings provide functional relevance, as athletes rarely land from heights greater than their maximal jumping capability. When task demand is equalized relative to lower extremity lean mass, the difference in absolute hip and knee energy absorption between sexes increases, but there is no effect on relative joint contributions to total energy absorption [15]. Therefore, absolute LHs potentially create inequitable task difficulty and expose participants to mechanical demands exceeding those faced in real-life, sport specific situations where most ACL injuries occur. Considering few studies have observed participants performing landings relative to maximal jumping ability [10], the purpose of this study was to examine sex differences in GRF, kinematics, kinetics, and energetics during unilateral landings from relative and absolute LHs. We hypothesized that females would not exhibit increased high-risk mechanics compared to males when landing from a height relative to their maximum jumping ability, but would during landings from absolute LHs due to the inequitable relative task demand. 2. Methods 2.1. Participants Prior to data collection, experimental procedures received ethical approval from the university's Institutional Review Board. Forty-four healthy, recreationally active individuals between 18 and 30 years of age volunteered to participate. Each volunteer provided written, informed consent and completed a background questionnaire to screen for health status prior to participation. Volunteers were accepted if they had no history of lower extremity injury requiring surgical repair, and had not suffered a lower extremity injury within the previous six months. Recreationally active was defined as being physically active at least three times per week for a minimum of 30 min. At least one of these activity sessions was required to include jumping and landing components (e.g., basketball, volleyball). Additionally, participants were required to be pain free in the lower extremity on testing days. All participants wore spandex shorts and standard laboratory footwear (Air Max Glide, Nike, Beaverton, OR). 2.2. Experimental protocol Participants' dominant leg was first determined as the leg which could kick a ball the farthest. Participants then completed three maximal effort countermovement jumps on a force plate (Bertec FP460, Columbus, OH) while GRF data were recorded at 2000 Hz with custom software (LabVIEW, v11.0, National Instruments Corporation, Austin, TX). Jump height was calculated using the impulse–momentum relationship, and participants' maximum vertical jumping ability was defined as the highest of three jumps. Retro-reflective markers were then placed on specific anatomical landmarks [10]. Markers used exclusively for the standing calibration trial were placed bilaterally on the acromioclavicular joints, iliac crests, greater trochanters, medial and lateral femoral epicondyles, medial and lateral malleoli, and the first and fifth metatarsophalangeal joints. Rigid plates with four retro-reflective marker clusters were attached to the torso and pelvis, as well as bilateral thighs, shanks, and heels of the shoes for segment tracking during motion trials. Once markers were attached in the proper locations, a three second standing calibration trial was collected. Calibration markers were removed and participants completed five successful unilateral landings

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on their dominant limb from heights of 30 cm (D30), 40 cm (D40), 50 cm (D50), and a height equal to their maximum jumping ability (DR). These absolute heights were chosen because they are commonly used to assess sex differences in unilateral landing mechanics [27]. Participants were instructed to keep their arms folded across their chest throughout the landing. While arm placement may alter landing mechanics [28], this position was chosen to remain consistent with previous research [10,11,29] and eliminate variability in landing mechanics due to arm motion. A successful trial was defined as participants' ability to perform the task without stepping down or jumping up from the box and their entire dominant foot landed on the force plate while refraining from hopping upon landing, touching down their contralateral foot or uncrossing their arms to help control the landing. Order of landing tasks was randomized. For all trials, marker coordinate data were collected at 200 Hz with an eight-camera motion analysis system (Vicon, Centennial, CO, USA), while GRF data were collected synchronously at 2000 Hz with the force plate.

2.3. Data analysis Data reduction and analysis were implemented with Visual3D (v5.00, C-Motion Inc., Rockville, MD). Raw three-dimensional marker coordinate and GRF data were low-pass filtered using a fourth-order, zero lag, recursive Butterworth filter with cutoff frequencies of eight and 50 Hz, respectively [30]. Right-handed Cartesian segmental coordinate systems were defined to describe trunk and pelvis, as well as bilateral thigh, shank, and foot position and orientation using an unweighted least squares procedure [31]. Three-dimensional hip, knee, and ankle angles were determined using a joint coordinate system approach [32]. Hip joint centers were placed 25% of the distance from ipsilateral to contralateral greater trochanter markers [33]. Knee joint centers were the midpoint between femoral epicondyle markers [32], and ankle joint centers were the midpoint between malleoli markers [34]. Three-dimensional joint kinetics were calculated using a Newton– Euler approach [35], and reported in the distal segment reference frame. Body segment parameters were estimated from Dempster [36]. Predefined GRF, kinematic, kinetic, and energetic variables were identified based on those suggested previously to impact ACL injury risk [5,7,10,15–17,37]. GRF variables included peak vertical GRF (VGRF) and posterior GRF (PGRF). Kinematic variables included hip flexion and adduction, knee flexion and adduction, as well as ankle plantarflexion and inversion at initial contact (IC), defined as the instant VGRF first exceeding 10 N. Kinetic variables included peak hip extensor and abductor, knee extensor and adductor, as well as ankle plantarflexor and eversion moments. Finally, energetic variables included hip, knee, and ankle sagittal plane net joint work, calculated during the landing phase by integrating the respective joint power curves. Joint moments and GRF were normalized to body mass times the square root of LH, because GRF during landing is approximately proportional to the square root of LH based on the impulse–momentum relationship and properties of uniformly accelerated motion [38]. Energy absorption at each joint was normalized by body mass times LH, because total mechanical energy is directly proportional to LH [10].

2.4. Statistical analysis Dependent variables were submitted to four (GRF, kinematic, kinetic, and energetic) separate 2 × 4 (sex × LH) MANOVAs in SPSS (SPSS v21.0, SPSS Inc., Chicago, IL). Follow-up univariate ANOVA was conducted in the event of significant MANOVAs (p b 0.05). Dependent variables that demonstrated significant univariate sex × LH interactions and LH effects were subsequently examined using post hoc pairwise comparisons with Bonferroni-adjusted p-values of p ≤ 0.003 and p ≤ 0.008, respectively. Significance for univariate sex effects was p b 0.05.

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Table 1 Effect of sex and landing height on mean ± STDV peak vertical (VGRF) and posterior (PGRF) ground reaction forces (N/kg−1/LH−½).

VGRF PGRF⁎

Females Males Females Males

D30

D40

D50

DR

77.0 ± 13.8 79.7 ± 11.4 −11.4 ± 1.7 −10.6 ± 1.5

75.6 ± 11.9 79.8 ± 10.6 −11.2 ± 2.7 −9.8 ± 1.6

77.0 ± 11.7 78.4 ± 9.8 −11.6 ± 2.8 −9.7 ± 1.5

75.1 ± 14.3 76.3 ± 10.1 −11.6 ± 1.8 −10.1 ± 1.5

⁎ Significant sex effect (p b 0.05).

7.77, p b 0.01, η2p = 0.17). However, neither the interaction, nor the main effects were significant for peak hip flexor or ankle eversion moments. Post hoc pairwise comparisons indicated that males exhibited decreased peak hip abductor moment during D30 landings compared to D50 and DR landings. At the knee, females exhibited greater peak extensor moments during DR landings compared to males. Females also displayed decreased peak knee extensor moments during D50 compared to DR landings, as well as increased peak knee abductor moments during D50 landings compared to DR and D30 landings. On the other hand, males only exhibited decreased peak knee extensor moments as LH increased, with no significant differences between the relative and any absolute LH. Lastly, at the ankle, females exhibited greater peak ankle plantarflexor moments during DR and D30 landings compared to males. Females also exhibited decreased peak ankle plantarflexor moments as LH increased, as well as during D50 compared to DR landings.

3. Results Three participants failed to complete the protocol and were excluded from all data analysis. The remaining participants included 21 females (height: 1.63 ± 0.05 m, mass: 61.32 ± 8.08 kg, DR: 27 ± 4 cm) and 20 males (height: 1.80 ± 0.05 m, mass: 80.35 ± 11.26 kg, DR: 43 ± 8 cm). 3.1. Ground reaction forces The sex × LH interaction was not significant (F(6,34) = 0.91, p = 0.44, η2p = 0.02) for GRF (Table 1). The main effect for LH was also not significant (F(6,34) = 1.21, p = 0.30, η2p = 0.18), but a significant sex effect was identified (F(2,38) = 4.03, p = 0.03, ηp2 = 0.18). Follow-up analyses identified that females exhibited increased PGRF compared to males. 3.2. Kinematics For joint kinematics (Table 2), a significant interaction was identified (F(18,22) = 2.56, p = 0.02, η2p = 0.68), along with a significant LH effect (F(18,22) = 16.39, p b 0.01, η2p = 0.93). However, the main effect for sex was not significant (F(6,34) = 1.22, p = 0.32, η2p = 0.18). Univariate analyses revealed a significant interaction for IC hip abduction (F(3,117) = 15.88, p b 0.01, η2p = 0.29). Post hoc pairwise comparisons indicated that females exhibited increased IC hip abduction during D50 landings, but decreased IC hip abduction during DR landings compared to males. Furthermore, both males and females exhibited significant increases in IC hip abduction as absolute landing height increased. Univariate analyses also revealed significant LH main effects for IC hip flexion (F(3,117) = 5.21, p b 0.01, η2p = 0.12), and IC knee abduction (F(3,117) = 3.26, p = 0.02, η2p = 0.58). Post hoc pairwise comparisons indicated that IC hip flexion was less during D50 landings compared to D30 landings, and IC knee abduction was greater during D50 landings compared to D30 and D40 landings. 3.3. Kinetics With regard to joint kinetics (Table 3), the interaction (F(18,22) = 4.05, p b 0.01, η2p = 0.77) and main effect for LH were significant (F(18,22) = 3.38, p b 0.01, η2p = 0.73), but the main effect for sex was not significant (F(6,34) = 1.55, p = 0.19, ηp2 = 0.21). Univariate analyses identified significant interactions for peak hip abductor (F(3,117) = 3.04, p = 0.03, η2p = 0.07), knee flexor (F(3,117) = 3.20, p = 0.03, η2p = 0.08), knee abduction (F(3,117) = 3.30, p = 0.02, η2p = 0.08), and ankle plantarflexor moments (F(3,117) =

3.4. Energetics The energetics MANOVA revealed a significant interaction (F(9,31) = 2.74, p = 0.02, η2p = 0.44) and a significant LH effect (F(9,31) = 20.84, p b 0.01, η2p = 0.86) (Table 4). The main effect for sex, however, was not significant (F(3,37) = 1.87, p = 0.15, η2p = 0.13). Significant interactions were identified through follow-up univariate analyses for knee (F(3,117) = 5.07, p b 0.01, η2p = 0.12) and ankle work (F(3,117) = 8.36, p b 0.01, η2p = 0.18). Conversely, neither the interaction, nor the main effects were significant for hip work. At the knee, females absorbed more energy during DR compared to D40 and D50 landings. Females also absorbed more energy during D30 compared to D50 landings. Males showed similar decreases in knee energy absorption as LH increased; however, they did not exhibit any significant differences between relative and absolute LHs. At the ankle, females also absorbed more energy during DR and D30 landings compared to D40 and D50 landings. On the other hand, males only exhibited increased ankle energy absorption during D30 landings compared to D50 landings.

4. Discussion The purpose of this study was to investigate the effects of sex and LH on GRF, kinematics, kinetics, and energetics during unilateral landings from relative and absolute LHs. Our results indicate that females exhibit decreased IC hip abduction, along with increased peak knee extensor and plantarflexor moments when landing from a height equal to their maximum jumping ability. Additionally, while both females and males utilize the knee as the primary energy absorber, females selected a landing strategy which emphasizes greater energy absorption from the ankle plantarflexor musculature than their male counterparts. Our results also indicate that females and males respond differently to increasing LH. Specifically, as LH increased peak knee adductor moment increased while peak ankle plantarflexor moment decreased in females. On the other hand, males displayed decreased peak hip abductor moments as LH increased. Interestingly, neither VGRF nor PGRF was altered by increasing LH. However, females consistently landed with greater PGRF compared to

Table 2 Effect of sex and landing height on mean ± STDV hip, knee and ankle joint angles (degrees) at initial contact. Positive angles represent hip flexion and adduction, knee extension and adduction, as well as ankle dorsiflexion and inversion.

Hip flexion

†1

Hip adduction

‡

Knee extension Knee adduction†12 Ankle dorsiflexion Ankle inversion † 1 2 ‡ § a b c

Females Males Females Males Females Males Females Males Females Males Females Males

Significant landing height effect (p b 0.05). D30 significantly different from D50 (p ≤ 0.008). D40 significantly different from D50 (p ≤ 0.008). Significant sex × landing height interaction (p b 0.05). Females significantly different from males (p ≤ 0.003). Significantly different from DR (p ≤ 0.003). Significantly different from D40 (p ≤ 0.003). Significantly different from D50 (p ≤ 0.003).

D30

D40

D50

DR

18.9 ± 12.6 16.2 ± 8.2 −10.3 ± 3.5bc −9.3 ± 3.8bc −13.5 ± 5.2 −15.6 ± 5.1 −0.0 ± 3.3 2.0 ± 4.8 −21.2 ± 7.3 −16.3 ± 9.7 8.2 ± 5.7 7.8 ± 5.8

16.8 ± 11.2 14.8 ± 8.6 −13.1 ± 8.5ac −11.4 ± 4.0c −13.6 ± 5.2 −14.7 ± 5.4 −0.1 ± 3.0 2.1 ± 4.2 −20.6 ± 9.5 −18.1 ± 9.1 7.7 ± 5.3 8.0 ± 5.9

15.7 ± 12.0 13.6 ± 8.0 −16.0 ± 4.2§a −13.7 ± 3.0 −13.8 ± 5.0 −15.0 ± 4.8 −0.9 ± 3.3 1.5 ± 4.5 −21.0 ± 6.2 −18.4 ± 8.6 7.6 ± 5.6 7.4 ± 6.8

18.0 ± 12.5 15.0 ± 7.1 −8.7 ± 4.0§ −12.0 ± 4.4 −13.7 ± 4.4 −15.9 ± 4.8 0.1 ± 3.3 1.9 ± 4.5 −20.2 ± 8.8 −18.7 ± 10.0 7.6 ± 5.8 8.1 ± 6.2

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Table 3 Effect of sex and landing height on mean ± STDV peak hip, knee and ankle joint moments (N/kg−1/m−1/LH−½) during the landing. Positive moments represent hip flexion and adduction, knee extension and adduction, as well as ankle dorsiflexion and inversion.

Hip flexion Hip adduction‡ Knee extension‡ Knee adduction‡ Ankle dorsiflexion‡ Ankle eversion ‡ § a b c

Females Males Females Males Females Males Females Males Females Males Females Males

D30

D40

D50

DR

−4.54 ± 1.74 −4.84 ± 1.18 −2.42 ± 0.75 −2.35 ± 0.74ac 3.04 ± 0.45 2.92 ± 0.51bc 0.43 ± 0.42c 0.41 ± 0.30 −2.21 ± 0.42§bc −1.80 ± 0.50 −0.11 ± 0.11 −0.10 ± 0.07

−4.83 ± 1.51 −4.94 ± 1.29 −2.36 ± 0.80 −2.28 ± 0.64 2.96 ± 0.38 2.73 ± 0.53 0.49 ± 0.45 0.42 ± 0.31 −1.98 ± 0.51 −1.80 ± 0.38 −0.09 ± 0.07 −0.10 ± 0.07

−5.26 ± 1.42 −5.02 ± 1.29 −2.41 ± 0.76 −2.10 ± 0.72 2.97 ± 0.44a 2.63 ± 0.45 0.52 ± 0.44a 0.48 ± 0.34 −1.89 ± 0.41a −1.76 ± 0.38 −0.09 ± 0.10 −0.10 ± 0.08

−4.64 ± 1.86 −4.93 ± 1.30 −2.47 ± 0.66 −2.18 ± 0.71 3.12 ± 0.42§ 2.76 ± 0.43 0.38 ± 0.33 0.46 ± 0.35 −2.22 ± 0.54§ −1.75 ± 0.37 −0.11 ± 0.11 −0.10 ± 0.09

Significant sex × landing height interaction (p b 0.05). Females significantly different from males (p ≤ 0.003). Significantly different from DR (p ≤ 0.003). Significantly different from D40 (p ≤ 0.003). Significantly different from D50 (p ≤ 0.003).

males. This finding is similar to previous literature that reported females display greater PGRF during bilateral landings [8], and stop-jump tasks [39]. While these tasks differ from the unilateral landing utilized in the current study, similar findings across the aforementioned studies suggest that greater PGRF is apparent in females across a variety of sporting movements. These combined results suggest the importance of PGRF as a risk factor and potential contributor to the sex disparity in ACL injury rates. Larger PGRF during landing may increase ACL loading by inducing an increased quadriceps muscle contraction and anterior shear force on the proximal tibia [40–43]. While, alterations in IC knee flexion angle will affect the quadriceps line of action [44], and consequently the magnitude of quadriceps shear force and ACL loading, males and females in the current study landed with similar knee flexion angles. Furthermore, females landed with greater peak knee extensor moments compared to males, which may be associated with greater quadriceps muscle forces. Thus, the observed larger PGRF exhibited by females, may have increased the anterior shear force on the proximal tibia induced by the quadriceps muscle, and thereby increased ACL loading [45,46]. However, it has been reported that sagittal plane knee joint forces alone cannot rupture the ACL as the interaction between muscle and sagittal plane joint mechanics and ground reaction forces places a ceiling on ligament loads [47]. Furthermore, ACL loading during dynamic tasks appears to be the result of a multifaceted interaction of sagittal plane shear forces, as well as frontal and transverse plane knee moments [19,49]. While females landed with greater peak knee extensor and ankle plantarflexor moments than males from heights equal to their maximum jumping abilities, there was not a sex difference in peak knee adductor moments during DR landings. Although knee adductor moments have been prospectively identified as a predictor of ACL injuries during bilateral drop jumps among adolescents [50], this finding is consistent with previous unilateral landing literature [10], and suggests that peak

knee adductor moments may be less relevant for ACL injury risk during unilateral tasks. Our cohort was also older than that of Hewett et al. [50], and individuals with such high knee adductor moments may have been previously injured or have other characteristics that excluded them from participating in the current study. Nevertheless, these combined results support our first hypothesis and efforts should be made to prospectively identify other potential risk factors during unilateral tasks in young adult athletes. The joint moment results also support our second hypothesis. Most notably, as LH increased, males exhibited a decreased peak hip abductor moment, whereas females exhibited an increased peak knee adductor moment and a decreased peak ankle plantarflexor moment. Dynamic knee valgus loading has been defined as the combination of externally applied hip adductor and knee abductor moments. In the current study, internal joint moments were reported, translating the definition of dynamic knee valgus loading to a combination of hip abductor and knee adductor moments. This combined loading has been identified as an ACL injury risk factor [50], and purportedly increases ACL strain [37]. Thus, increased peak knee adductor moment exhibited by females in conjunction with decreased peak hip abductor moment exhibited by males as LH increased is congruent with current theory regarding mechanisms underlying the sex disparity in ACL injury rates [5]. However, it is important to note that DR for females was 27 cm. Therefore, landings performed from 40 and 50 cm may have placed an unrealistic mechanical demand on females. Future studies are warranted to determine how increasing mechanical demands affects ACL injury risk and explore the link between morphological–mechanical interactions and resultant ACL loading. The extent of energy absorption during landing reflects the level of mechanical demands placed on the lower extremity and the corresponding mechanical responses of lower extremity musculature [25, 51]. Increased sagittal plane energy dissipation would be beneficial as

Table 4 Effect of sex and landing height on mean ± STDV hip, knee and ankle joint work (J/kg−1/LH−1) during the landing phase. Negative work values represent energy absorption.

Hip work Knee work‡ Ankle work‡ ‡ § a b c

Females Males Females Males Females Males

Significant sex × landing height interaction (p b 0.05). Females significantly different from males (p ≤ 0.003). Significantly different from DR (p ≤ 0.003). Significantly different from D40 (p ≤ 0.003). Significantly different from D50 (p ≤ 0.003).

D30

D40

D50

DR

−1.71 ± 1.06 −1.79 ± 1.05 −4.08 ± 1.18c −4.33 ± 1.05bc −2.88 ± 0.69bc −2.29 ± 0.93c

−1.63 ± 0.93 −1.49 ± 0.78 −3.68 ± 0.98 −3.71 ± 0.81 −2.32 ± 0.76a −1.99 ± 0.66

−1.57 ± 1.08 −1.52 ± 0.82 −3.47 ± 0.94a −3.42 ± 0.67 −2.06 ± 0.53a −1.85 ± 0.64

−1.91 ± 1.15 −1.59 ± 0.94 −4.43 ± 1.33 −3.84 ± 0.93 −3.08 ± 1.22§ −2.01 ± 0.81

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the lower extremity joints are able to provide adequate shock absorption without over-stressing the vulnerable soft tissue structures such as the ligaments [51]. In the current study, both females and males exhibited decreased knee and ankle energy absorption, with no change in hip energy absorption, as absolute LH increased. The comparable response exhibited by females and males to increased LHs is congruent with previous findings [15], but the absence of a shift towards a more proximal energy absorption strategy as LH increased was unexpected. Since kinetic energy at impact increases as LH increases, the observed alteration in energy absorption strategy in the current study indicates that passive structures absorbed more energy and were exposed to greater stress [52]. When landing from a height equal to maximum jumping ability, both females and males relied on the knee as the primary energy absorber which is consistent with previous literature [10]. The results of the current study also indicate that females absorbed more energy at the ankle than males during DR landings. This result is in agreement with previous research [11,13,25], which showed that females typically emphasize greater energy absorption from the ankle plantarflexor musculature than their male counterparts during landing. This distally dominant joint energy absorption strategy may expose proximal passive structures to higher forces during landing, and has been associated with ACL injury risk [16]. There are several methodological limitations that must be taken into account when interpreting results of the current study. First, joint kinematics were estimated via a skin-based marker system and associated motion artifact may not definitively reflect underlying bone translations and rotation. These errors are further propagated by the estimation of joint forces and moments derived via inverse dynamics. Second, this study investigated normal landing techniques, and thus, injurious performances and subsequent injury risk cannot be truly assessed. Third, the study was performed in a controlled laboratory environment where participants knew exactly what to expect. Although this allows accurate comparisons between conditions, it does not closely simulate athletic competition. Furthermore, the landing task itself was not identical to movements observed during athletic participation since we constrained arm use during the landings. As changing hand placements during landing alters landing mechanics [28], future studies should investigate the influence of arm position and motion on ACL injury risk. Finally, all participants were recreational athletes. Skill level has been identified as a confounding factor in predisposing females to ACL injury [2]. It was assumed that skill levels were not a cause of the observed differences presented in the current study. Although we cannot completely discount skill level as a factor that caused the apparent differences, male and female volunteers were screened for recreational sports participation that included landings. 5. Conclusion Our results indicate that females landed with greater PGRF compared to their male counterparts, and when landing from a height equal to their maximum jumping ability, both females and males utilize the knee as the primary energy absorber. Yet, females appear to select a landing strategy which emphasizes greater energy absorption from the ankle plantarflexor musculature than their male counterparts [10,13]. Our results also indicate that females and males respond differently to increasing LH. Specifically, females displayed an increased peak knee adductor moment while males displayed a decreased peak hip abductor moment as LH increased. This finding is consistent with current theory regarding frontal plane joint loading patterns and the sex disparity in ACL injury rates. However, D40 and D50 landings may have represented an unrealistic mechanical demand for females and influence subsequent inferences regarding ACL injury risk during landing. We therefore suggest that comparisons between studies utilizing different LHs be made with caution and participants jumping ability be taken into account whenever possible.

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