Synergic control of vertical body oscillation during stance phase of treadmill running

BACKGROUND: Stabilization of the vertical body oscillation is important to running efficiency and performance and might be affected by running speed. AIM: We used the uncontrolled manifold (UCM) framework to test whether there would be a multi-joint synergy stabilizing the vertical body oscillation on stance phase of treadmill running and the effect of running speed. METHOD: Experienced runners (N=28; 22-51 years old) ran on an instrumented treadmill set at 2.5, 3.5, and 4.5 m/s. Ankle, knee, and hip angles in the sagittal plane and pelvis obliquity were used as elemental variables (DOF=4) to calculate the synergy index (DVZ), which is the normalized difference between the variances in the joint space that did not affect (VUCM) and affected (VORT) the vertical body oscillation (i.e., center of mass) during the stance phase (1-100%). Statistical parametric mapping (SPM) analyses were performed to detect the presence of synergy and the effect of running speed. RESULTS: SPM analyses showed that independent of running speed, synergy was always present (DVZ>0), except between 13% and 17% in the slowest speed. Also, results showed that the synergy index was unaffected by speed. Additionally, while no effect of speed was seen in the VORT, VUCM in the fastest running speed was the highest between 7 and 43% of the stance phase. INTERPRETATION: The findings indicated the existence of a synergy stabilizing the vertical body oscillation during the stance phase in running and the running speed did not affect the stabilization of the vertical body oscillation during the stance phase.