Perturbed Gait Reveals Hierarchical Muscle Control: Reorganisation of Co-contraction Shares of Distal Muscles Amid Stable Global Synergies.

Adaptive gait control under external perturbations requires coordinated modulation of both local muscle activation and global muscular organisation. To capture these multi-level adaptations, this study integrates co-contraction share analysis, which characterises distributed muscle contributions beyond conventional antagonist-pair co-activation indices, with Principal Component Analysis (PCA) as an analytical framework for examining Electromyographic (EMG) responses to frontal-plane angular perturbations during overground walking. EMG signals are recorded from 10 subjects on distal and proximal lower-limb muscles during the Pre-Perturbation Stride (PPS), Perturbation Stride (PS), and Subsequent Stride (SS) under four perturbation conditions: Midstance Ipsilateral (MI), Midstance Contralateral (MC), Touchdown Ipsilateral (TI), and Touchdown Contralateral (TC). Co-contraction share is quantified using summed values, peak values, and peak locations across defined gait phases, while PCA is used to characterise synergistic activation patterns. Perturbation-related modulation is observed across all strides with effect magnitudes ranging from moderate to very large. PPS generally shows more stable activation patterns. PS exhibits the strongest perturbation-induced changes, whereas SS shows either partial recovery or persistence of altered activation depending on muscle group and perturbation condition. Distal muscles demonstrate the largest and most sustained effects, particularly in co-contraction magnitude and timing, indicating a dominant role in immediate stabilisation and post-perturbation recovery. Proximal muscles exhibit earlier, but generally smaller adjustments, consistent with whole-body coordination. PCA reveals fewer significant changes than co-contraction share analysis, possibly reflecting both preservation of global coordination structure and lower sensitivity to localised perturbation effects. These findings support a hierarchically organisation of perturbation responses and demonstrate the value of combining co-contraction share analysis with PCA for characterising muscular adaptation during perturbated gait. The identified control strategies may provide useful design principles for adaptive locomotion control to perturbation in bio-inspired legged robotic and wearable assistive technologies.