Biomechanical effects of passive exosuit assistance on tibiofemoral loading and dynamic stability during downhill walking.

Downhill walking demands significant eccentric muscle activity at the knee, which increases tibiofemoral joint contact force (TFCF) and challenges gait stability. However, it remains unclear whether passive exosuits can reduce TFCF during downhill walking without compromising dynamic stability. This study investigated the biomechanical effectiveness of a passive trunk-to-thigh soft exosuit in decreasing TFCF and examining its effects on dynamic stability during downhill walking. Fourteen healthy male participants walked on an instrumented treadmill set at a - 15° decline at 1.0 m·s⁻¹ under both unassisted and assisted conditions. Knee joint kinetics, TFCF, and dynamic stability parameters were estimated using a subject-specific full-body musculoskeletal model. Statistical Parametric Mapping (SPM) and paired t-tests were used to analyze continuous waveforms and discrete metrics, respectively. The exosuit decreased biological stance phase TFCF impulse by ~7% (axial) and ~6% (resultant). Notably, participants adopted shorter, higher-cadence, and wider steps with the exosuit, suggesting that the reduced loading was driven concurrently by both the device's direct support and user-driven gait adaptations. SPM identified a significant decrease in knee joint power generation during mid-stance (23-29% gait cycle (GC)). Additionally, the ML margin of stability (MoS) significantly increased by 7.4%, while the anteroposterior (AP) MoS remained unchanged. These results indicate that the current passive assistance can reduce cumulative tibiofemoral loading during downhill walking. Concurrently, the increased lateral margin of stability reflects a more cautious, wide-based gait strategy. These outcomes demonstrate that the device reduced joint loading through an intertwined combination of direct mechanical resistance and the induction of a joint-protective, shorter-step gait strategy. Consequently, passive wearable devices represent a promising approach for joint load management during downhill walking.