Multi-Joint Synergy in Foot Height Stabilization Across Different Running Speeds: An Uncontrolled Manifold Analysis

The uncontrolled manifold (UCM) framework was employed to investigate the presence of a multi-joint synergy stabilizing foot height during the swing phase of treadmill running and its potential dependence on running speed. Experienced runners (N = 28; aged 22-51) ran on an instrumented treadmill set at three different speeds: 2.5, 3.5, and 4.5 m/s. Kinematic data were utilized to calculate UCM outcomes: variances in the joint space that had no effect (V_UCM) and those that influenced (V_ORT) foot height, and the synergy index (ΔV_Z, the normalized difference between V_UCM and V_ORT). They were computed for each normalized frame (1-100%) of the swing phase and averaged in 10% intervals. ΔV_Z was greater than zero and varied across the swing phase, being lowest at 51-60% and highest at 81-100%. ΔV_Z was the lowest at the slowest speed in the second half of the swing phase, because of a low V_UCM. The findings indicate that the CNS organizes a multi-joint synergy to stabilize foot height, with the strongest synergy at the end of the swing phase to ensure safe foot placement and landing. Faster running speeds enhance this synergy, allowing greater adaptability to perturbations, while slower speeds lead to a more cautious approach, reducing overall variance.