Comparative analysis of muscle coordination patterns underlying different types of stepping movements.

Reactive stepping is crucial for preventing falls after losing balance. While perturbation-based training improves reactive step quality, voluntary step training appears less effective. To gain insight into the neural underpinnings of such task-specific effects, we examined the muscle coordination patterns of voluntary and reactive stepping. As an additional step type, we introduced action observation with motor simulation of reactive steps, as it has shown promise for improving reactive step quality without requiring real balance perturbations. Electromyographic signals were recorded from eight leg and trunk muscles of healthy young subjects (n = 15) during three step types: (1) reactive stepping following support-surface translations, (2) voluntary stepping in response to a visual stimulus, and (3) action observation with motor simulation of reactive steps, as demonstrated by a human actor. Each condition involved stepping with the right leg in five directions (anterior/45°anterior/lateral/45°posterior/posterior). Muscle synergy analysis was employed to identify muscle weights with corresponding temporal activation profiles, which were compared across step types. Step characteristics and body configurations at foot down were also compared. Three muscle synergies were consistently recruited across participants and step types. In reactive stepping, a majority of participants exhibited a fourth muscle synergy involving rectus femoris and soleus. Temporal activation coefficients and body configurations varied with step type. While largely similar muscle weights were found for the three types of stepping movements, higher levels of activation in reactive stepping presumably reflect the greater biomechanical challenge involved. These findings may help explain differences in effects between different step training protocols.