Lower Limb Neuromuscular Responses to Waist-Pull Perturbations during Reactive Stepping

Maintaining balance after unexpected perturbationsrequires rapid neuromuscular responses coordinated throughsensorimotor processes. Surface electromyography (sEMG) iswidely used to assess muscle activation patterns during suchresponses, but its interpretation can be limited by cross-talkand electrode placement sensitivity. To provide complementaryinsight, we combined sEMG with B-mode ultrasound (US), whichenables direct measurement of muscle architecture, specificallyfascicle length (FL) and muscle thickness (MT), during reactivestepping induced by anterior and posterior waist-pull pertur-bations. Eleven healthy adults were exposed to low- and high-amplitude waist pulls while standing on a force plate, withneuromuscular activity (using sEMG and US) recorded fromthe gastrocnemius (GAS) and soleus (SOL) of the stepping(dominant) leg, alongside joint kinematics and ground reactionforces. High-magnitude perturbations elicited greater sEMGactivity in GAS and SOL, increased MT in GAS only, and greaterFL shortening in GAS compared to low perturbations. A negativecorrelation between sEMG and FL (R = \u20130.73, p = 0.011),was observed in the medial GAS (MGS) during anterior\u2013highperturbations, while correlations for other conditions were weakto moderate (R= 0.12 \u2212 0.50, p = 0.12\u20130.72). MT showed nomeaningful correlation with sEMG. Adaptation analyses furtherrevealed parallel trends between sEMG and FL across repeatedtrials. Results showed that: 1) FL is a more reliable morphologicalindicator of neuromuscular responses than MT in the GAS;and 2) US imaging is a viable method for capturing musclearchitecture dynamics during rapid reactive stepping. Thesefindings may inform the development of assistive devices for fallprevention and targeted rehabilitation strategies.