Sex Dominates Task Anticipation in Shaping 90° Sidestep Cutting Biomechanics: Evidence for a Proximal-to-Distal Cascade

Proximal Contributions: Trunk-Pelvis-Hip

Women demonstrated a distinct multiplanar proximal configuration, characterized by a more upright trunk and pelvis, reduced hip and knee flexion, and altered pelvic and hip kinematics in the frontal and transverse planes. This reflects a stiffer, quadriceps-dominant pattern and reduced engagement of posterior-chain musculature, ³⁰ limiting load attenuation during early stance. Men, by contrast, demonstrated greater trunk flexion and ipsilateral lean, alongside higher hip internal rotation and knee external rotation moments. Consistent with previous work, ²¹ men also produced greater hip adduction moments across stance. For frontal-plane knee moments, SPM identified significant sex differences during early-, mid-, and late-stance, with men demonstrating larger varus-valgus-varus knee moment excursions. Greater hip internal rotation at initial contact during anticipated 90° versus 45° SSC has been suggested to reduce knee loading. ¹⁵ In the present study, men exhibited greater hip internal rotation across approximately 60% of stance, but frontal-plane knee moment differences did not indicate a corresponding reduction in knee loading. This suggests that the relationship between hip transverse-plane motion and frontal-plane knee loading may depend on task constraints and sex.

Women also showed reduced hip and knee flexion angles and lower hip extension and external rotation moments compared with men, paralleling patterns reported in anticipated or unanticipated ~45° SSC.^21,35,40 These findings align with the hypothesis that weak or delayed activation of hip extensors has been linked to upright, stiff landings, greater quadriceps reliance, and higher ACL risk.^16,23,30,35 By contrast, men displayed greater hip flexion angle and extension moments, accompanied by greater trunk flexion, consistent with a more hip-dominant sagittal-plane strategy. Such strategies have been linked to braking and redirection capacity in change-of-direction tasks. ¹⁸

Although both sexes maintained ~20° of hip abduction during stance, men generated greater internal hip abduction moments, indicating stronger gluteal recruitment for frontal-plane stability. In women, lower hip abduction moments, together with reduced anterior pelvic tilt and greater contralateral drop and rotation, may suggest less effective hip-abductor control and center-of-mass-GRF misalignment. These deficits, linked to femoral adduction, dynamic valgus, and tibial external rotation, ³⁹ align with prospective evidence indicating pelvic drop and frontal-plane asymmetries as predictors of noncontact ACL injury risk. ⁹ They also extend observations from anticipated 45° SSC by demonstrating that reduced hip abduction also occurs during the high-risk early postlanding phase. ³⁴ Musculoskeletal modeling further implicates non-knee-crossing musculature in resisting external valgus loads and offsetting ACL strain during unanticipated 45° SSC. ²³

Knee-Focused Mechanics

Internal varus moments are often cited as ACL risk surrogates because they rise in response to external valgus loads imposed by the GRF lever arm. Yet kinetics alone are insufficient; interpretation requires joint posture and proximal kinematics to capture true kinematic-kinetic coupling. Modeling studies have shown that combined valgus and internal rotation moments elevate ACL strain, ⁴¹ but these effects are posture-dependent: valgus torques near extension and hip adduction impose greater stress than the same loads under flexed, hip-dominant conditions. These findings should be interpreted as mechanistic contributors rather than direct proxies for injury in real world. For example, women in anticipated 45° SSC produced greater internal knee adduction moments during early deceleration despite the knee remaining in adduction (i.e., not valgus), ⁴³ highlighting that joint load arises from the GRF lever arm shaped by adjacent segments - not from knee kinetics alone.

Our findings extend this evidence. Consistent with previous study, ²¹ men showed a distinct early-stance knee kinetic pattern, larger net flexion, varus and external-rotation knee moment patterns alongside greater knee flexion and reduced valgus angles - features consistent with effective compensation. By contrast, as stance progressed, women shifted into greater valgus postures despite elevated varus moments, with reduced flexion, higher extension demand, and more internal rotation. This multiplanar coupling aligns with known ACL strain mechanisms, particularly when unopposed internal tibial torque develops at initial contact. ³ Coupled with reduced hip abduction, this may reflect weaker frontal-plane torque transmission from hip to knee and greater susceptibility to dynamic valgus collapse. ³⁸

Task and foot-to-ground interactions further modulate these dynamics. In women performing anticipated 45° SSC, a rearfoot strike with posterior center-of-pressure shifts the GRF vector toward combined external valgus and internal-rotation moments - a multiplanar loading profile linked to ACL injury. ²⁸ In our 90° SSC, increased geometric and inertial demands plausibly magnifying GRF lever arms even when knee posture remained near neutral in the frontal plane. In this context, the larger internal- and external-rotation moments observed in men likely reflect resistive responses to elevated external demands rather than intrinsically risky states. Thus, task angle and strike pattern can generate risky frontal-transverse loading profiles even without visible valgus. The key determinant of risk is whether these exposures co-occur with vulnerable joint positions such as reduced knee flexion or dynamic valgus, and insufficient neuromuscular resistance to rapid multiplanar knee loading.^6,17,19,28 Taken together, these findings underscore that knee joint moments must be interpreted in light of multiplanar coordination and whole-body posture. Identical kinetic signatures may reflect either protective compensation or residual risk depending on the surrounding control strategy.

Distal Contributions

Distal mechanics play a critical yet often underappreciated role in shaping proximal joint loading during high-demand tasks like SSC. Studies of ~45° SSC have linked toe-in foot positions to greater knee abduction angles and moments under anticipated conditions, ³³ and to sex-specific ankle strategies under unanticipated conditions, with men shifting from early inversion to late eversion, whereas women sustained eversion throughout stance. ²¹ In our 90° SSC, participants adopted externally rotated foot postures, with women doing so to a greater extent - likely reflecting greater mechanical demand. Women also demonstrated greater early inversion and midstance dorsiflexion, but reduced internal-rotation, eversion, and plantarflexion moments. These patterns suggest limited capacity to generate stabilizing torques at the ankle, particularly in the frontal and transverse planes. Consistent with a recent anticipated 90° SSC study, ⁸ women also produced smaller vertical and mediolateral GRFs in late stance, suggesting a different push-off strategy with implications for late-stance force generation and compromised multiplanar control. ⁴⁵

Proximal-to-Distal Adaptation Sequence

Anticipation and dual-task demands have been shown to influence ACL-related knee mechanics, with individual studies reporting greater peak abduction moments and flexion angles under cognitively challenging conditions. ¹³ However, pooled findings of different studies remain inconclusive - likely due to variability in cut angles (30°-90°) and approach speeds (≈2.78-5.37 m/s). ⁴⁸ Our results align partially with these findings, confirming greater ipsilateral trunk tilt during unanticipated 90° SSC, but the sharper 90° cut angle and increased mechanical demands with less aggressive change-of-direction tasks caution against direct extrapolation.

A key finding was that unanticipated cutting did not simply alter isolated knee variables, but reorganized movement and loading demands across stance in a proximal-to-distal sequence. Early trunk flexion and reduced pelvic mobility were consistent with a more stabilizing proximal response during load acceptance under reduced preparatory time, but this appeared to shift demands distally as stance progressed. This interpretation was supported by the extent and duration of significant SPM intervals at the trunk, pelvis, and hip, indicating sustained proximal adaptations rather than isolated peak differences. During mid- to late-stance, significant SPM differences became more apparent at the hip and knee in the frontal and transverse planes, whereas the ankle showed the clearest late-stance adjustments during push-off. Together with the attenuated braking forces, this suggests that unanticipated trials limited effective pre-planned deceleration and required more of the redirection task to be managed within stance itself. This shift is important clinically because it suggests that unanticipated 90° cutting is not only a knee-loading problem, but a time-constrained whole-body coordination challenge.

Overall, these findings reinforce that unanticipated 90° SSC should not be interpreted through isolated joint metrics alone. Instead, the task appears to challenge how deceleration, redirection, and push-off are coordinated across stance, with particularly pronounced late-stance adjustments at the ankle. From an applied perspective, this supports assessment and training approaches that emphasize whole-body multiplanar coordination rather than isolated knee-focused peak values.

This study should be interpreted considering several limitations. Our laboratory-based mechanistic insights represent partial surrogates of in-game risk. ¹⁰ SSC dynamics vary with approach speed, angle, and external constraints, and our findings from a 90° cut may not generalize across all contexts. ¹⁰ The cohort comprised competitive but nonelite athletes, limiting extrapolation to professionals. Sex differences were examined only biomechanically, without consideration of anatomical or hormonal influences. Environmental influences - such as surface properties, traction, and game context (opponent pressure, decision speed, dual-tasking) - were not replicated but likely exacerbate risky mechanics. In addition, the inverse dynamics approach assumes rigid-body segments and does not account for soft tissue artifacts, which may affect moment estimation. Analyses were restricted to the dominant limb; as asymmetries could exist even in healthy young athletes and many ACL injuries occur on the nondominant side, bilateral assessment would provide a more complete picture.