Multi-joint synergies stabilize anteroposterior and vertical toe clearance during single-step ascent

Single-step ascent (SSA) requires precise control of foot trajectory, as insufficient or excessive toe clearance may increase tripping risk, compromise balance, or reduce movement efficiency. We expected that anteroposterior and vertical toe trajectories would be stabilized by multi-joint synergies, particularly at three critical events: anteroposterior clearance (AP_C), vertical clearance (V_C), and maximum toe height (H_MAX). The aims of this study were to (i) determine the minimum number of trials required for reliable Uncontrolled Manifold (UCM) outcomes, (ii) test whether multi-joint synergies stabilize toe position at AP_C, V_C, and H_MAX, and (iii) compare synergy strength across these events. Sixteen healthy adults (20-40 years) performed 60 SSA trials onto a 17-cm-high step. Kinematic data were used to compute toe trajectory and joint angles (ankle, knee, hip, and pelvis). Synergies were quantified using the UCM framework by partitioning variance into components within the UCM (V_UCM) and orthogonal to it (V_ORT), and computing the synergy index (ΔV_Z). Approximately 40 trials were required to obtain reliable estimates. ΔV_Z values were significantly higher than zero at all three events (AP_C: 0.39 ± 0.27; V_C: 0.50 ± 0.32; H_MAX: 0.85 ± 0.43), indicating stabilization of toe position. Synergy strength was greater at H_MAX, driven by increased V_UCM, while V_ORT remained unchanged. These findings indicate that the central nervous system stabilizes toe position during SSA through multi-joint synergies, with event-dependent modulation of coordination.