Effects of first-step constraints on multi-muscle synergies and early postural adjustments during step initiation

Step initiation requires the coordinated preparation of posture and movement, yet how first step constraints influence multi-muscle synergies stabilizing the center of pressure (COP), anticipatory synergy adjustments (ASAs), and early postural adjustments (EPAs) remains poorly understood. This study examined the effects of stepping surface height and step length on these outcomes in healthy young adults. Sixteen participants initiated a step under five conditions: level, step-up, step-down, short-step, and long-step. Surface electromyography was recorded from 16 leg and trunk muscles bilaterally, and COP coordinates were obtained from a force platform. The uncontrolled manifold approach was used to quantify multi-muscle synergies and ASAs stabilizing COP in the anteroposterior (AP) and mediolateral (ML) directions during steady-steady and around the foot lift-off {-25:+25 ms}. Multi-muscle synergies stabilizing COP-AP were present across all conditions, and their index did not differ significantly across stepping conditions, supporting the view that steady-state synergic organization reflects the demands of upright stance rather than anticipation of the upcoming step. Synergies stabilizing COP-ML were absent in all conditions. ASAs were not detected in any condition or direction, suggesting that step initiation relies more on EPA-based rather than synergy-level anticipatory mechanisms. Stepping surface height significantly influenced EPA onset timing and COP displacement magnitude in both directions, with the step-down condition producing earlier EPA onset and smaller displacements than the step-up condition. Step length did not significantly affect EPAs. These findings reveal a dissociation between synergic muscle organization and COP trajectory control and highlight the asymmetric postural demands of descending versus ascending steps during step initiation.