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

Area of Science:

• Biomechanics
• Motor Control
• Human Movement Science

Background:

• Single-step ascent (SSA) demands precise foot trajectory control to prevent tripping and maintain balance.
• Multi-joint synergies are hypothesized to stabilize critical foot clearance events during SSA.

Purpose of the Study:

• Determine reliable Uncontrolled Manifold (UCM) outcome trial numbers.
• Investigate multi-joint synergy's role in stabilizing toe position at anteroposterior clearance (APc), vertical clearance (Vc), and maximum toe height (HMAX).
• Compare synergy strength across these critical SSA events.

Main Methods:

• Sixteen healthy adults performed 60 SSA trials onto a 17-cm step.
• Kinematic data captured toe trajectory and joint angles (ankle, knee, hip, pelvis).
• The Uncontrolled Manifold (UCM) framework quantified synergies via variance partitioning (VUCM, VORT) and synergy index (ΔVZ).

Main Results:

• Approximately 40 trials were needed for reliable UCM estimates.
• Significantly positive ΔVZ values at APc, Vc, and HMAX confirmed toe position stabilization by synergies.
• Synergy strength was greatest at HMAX, driven by increased VUCM, with stable VORT.

Conclusions:

• Multi-joint synergies effectively stabilize toe position during single-step ascent.
• The central nervous system modulates coordination through event-dependent synergy strength.
• Findings highlight the neural control mechanisms underlying safe and efficient stair climbing.