"Heel Height and Wearing Experience Shape the Organization of Lower-Limb Muscle Synergies During Reactive Balance Perturbations"

"Abstract\u2014High-heeled footwear imposes persistent biomechanical constraints on ankle mechanics, yet whether long-term wearing experience alters the neural coordination architecture underlying reactive balance remains unclear. This study quantified the effects of heel height and wearing experience on lower-limb muscle synergies during perturbation-based standing balance. Thirty-nine healthy young females were assigned to an Experienced group (n = 21) or Control group (n = 18) based on high-heel wearing frequency. Participants performed translational (Motor Control Test) and rotational (Adaptation Test) perturbations while wearing shoes with four heel heights (0.8, 3.9, 7.0, and 10.1 cm). Surface electromyography from eight trunk and lower-limb muscles was decomposed using non-negative matrix factorization. Functional clustering, cosine similarity (>0.7 threshold), and statistical parametric mapping quantified spatial and temporal characteristics of muscle synergies across tasks and heel conditions.Both groups exhibited two to four synergies per condition. Inter-group similarity was limited, with only three synergy pairs exceeding the similarity threshold, indicating distinct coordination architectures under identical perturbations. In contrast, intra-group similarity across heel heights was markedly greater in the Experienced group (26 significant pairs) than in the Control group (5 pairs), with 69% of similarities occurring during forward perturbations. Despite these coordination differences, postural stability measures showed no significant group effects. Temporal activation patterns of shared synergies were largely preserved, except for localized differences at 10.1 cm during large backward perturbations. These findings demonstrate experience-dependent reorganization of modular neuromuscular control without detectable changes in behavioral stability, highlighting the utility of synergy-based system analysis for investigating adaptive motor control under altered biomechanical constraints."