Independent control of force and timing symmetry in dynamic standing balance: Implications for rehabilitation of hemiparetic stroke patients

Abstract

The extent to which force and timing are independently controlled has aroused research interest. The present study aims to examine the relationship between force and timing symmetrical performances during repetitive body weight shifting. Eight hemiparetic stroke patients and eight age- and gender-matched controls were required to repetitively shift weight from one leg to the other while standing with each foot on a separate force plate. Instructions emphasised symmetry of weight transfer in each direction. No knowledge of result feedback was given. The ratio of the vertical ground reaction forces between the legs (force symmetry) was contrasted with the ratio of the intervals between leftward and rightward movements (timing symmetry). The patients showed more asymmetric and variable performances than the controls. Timing symmetry was more variable than force symmetry. In both groups, force and timing symmetry were uncorrelated. Autocovariance estimates revealed persistent alternation of large and small ground reaction forces, whereas a tendency for alternation between short and long intervals rapidly damped to zero. In conclusion, both healthy and stroke participants exhibit independent control of the force and timing symmetry under the task of natural weight shifting. This may explain the weakness of current balance retraining paradigms based on visual biofeedback, in which an emphasis on force symmetry fails to help timing performance in dynamic tasks.