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 (APC), vertical clearance (VC), and maximum toe height (HMAX). 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 APC, VC, and HMAX, 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 (VUCM) and orthogonal to it (VORT), and computing the synergy index (ΔVZ). Approximately 40 trials were required to obtain reliable estimates. ΔVZ values were significantly higher than zero at all three events (APC: 0.39 ± 0.27; VC: 0.50 ± 0.32; HMAX: 0.85 ± 0.43), indicating stabilization of toe position. Synergy strength was greater at HMAX, driven by increased VUCM, while VORT 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.