Adaptive Changes in Lower-Limb Muscle Activations During Repeated Trip-like Perturbations in Young Adults

Background: Falls are a leading cause of injury and mortality worldwide. Higher physical activity levels in young adults may increase exposure to fall-related situations. Understanding their neuromuscular adaptations is critical for balance control research and perturbation-based training. This study examined proactive and reactive adaptations in lower-limb muscle activity during repeated simulated trips among young adults. Methods: Twenty participants experienced five treadmill-induced standing-trips. Bilateral electromyography (EMG) activities of the rectus femoris (RF), vastus lateralis (VL), tibialis anterior (TA), medial gastrocnemius (MG), and biceps femoris (BF) were recorded. Muscle activity magnitude at perturbation onset (ON), EMG peak amplitude, and time-to-peak from ON were extracted and compared across trials. Results: Proactive activation at ON increased across trials in TA and RF on the recovery side (p = 0.012–0.023) and in TA, VL, and BF on the stance side (p = 0.002–0.034). Reactive peak amplitudes decreased in RF, VL, and BF on the recovery side (p < 0.001–0.014) and in RF, VL, and BF on the stance side (p < 0.001–0.016). Time-to-peak shortened in MG, RF, VL, and BF on the recovery side (p < 0.001–0.030) and in RF, VL, TA, and BF on the stance side (p < 0.001–0.050). Conclusions: Repeated simulated trips elicited proactive adaptations in muscle activity and reactive changes in time-to-peak, which may suppress the need for increased reactive muscle activations to recover balance post-perturbation over trials in young adults. The findings augment our understanding of the intercorrelation between proactive and reactive adaptations to repeated perturbations.