Constraints on Joint Degrees of Freedom Affect Human Postural Dynamics: A Pilot Study

Abstract

This study aimed to investigate the direct relationship between the joint degrees of freedom (DoF) of human movement system and its postural dynamics. In our pilot experiment we fixed the join DoF (knee and ankle) to constrain the functional DoFs (one for knee, two for ankle). Young healthy participants were required to perform the single-leg standing task with their dominant leg fixed. The center of pressure (COP) trajectory data were measured and analyzed using linear and nonlinear methods to assess static and dynamic property of their postural dynamics. Results of comparing across conditions (normal no-fixation, ankle and knee fixation condition) revealed that static measure (COP trajectory length) did not differ significantly. However, dynamic measures (sample entropy and the fractal scaling exponent) significantly differed. The ankle joint fixation affected sample entropy decline (losing efficiency of postural control) and the scaling behavior (weakening the anti-persistent postural control process) in the mediolateral direction. These results seemed to agree with the notion of the loss of complexity framework.