Variability of reciprocal aiming movements during standing: the effect of amplitude and frequency

This study investigated the variability of the center of pressure (COP) trajectory during voluntary whole-body oscillations. While standing upright on a force platform, eight subjects leaned forward and backward so as to perform reciprocal aiming movements with their COP at a prescribed frequency (F) and amplitude (A) using online visual feedback of their COP location. A total of 25 F-A combinations were tested for each subject (3 < A < 9 cm, and 0.35 < F < 1.35 Hz). Spatial and temporal variability of the COP was assessed by computing the standard deviation (SD) and coefficient of variation (CV) of the amplitude (trough to peak) and frequency (peak to peak) of the COP cycles within each trial, respectively. The results revealed that all variability indices depended on the prescribed F and A. Concerning the effect of spatial constraints on spatial variability, SD spatial increased as a function of A, while CV spatial decreased as function of A. A similar pattern was observed with respect to the effect of temporal constraints on temporal variability (SD temporal increased as a function of F, while CV temporal decreased). As for "cross-over" effects, there was an effect of F on spatial variability, such that SD spatial and CV spatial were minimal at 0.6 Hz. For the "cross over" effect of A on temporal variability, both SD spatial and CV spatial decreased as a function of A. Across the experimental conditions, there were weak or no correlations between variability in the time and space domain. Comparisons with an earlier study on human gait (Danion F, Varraine E, Bonnard M, Pailhous J. Stride variability in human gait: the effect of stride frequency and stride length. Gait Posture 2003;18:69-77) suggest that the effects of spatial constraints are relatively task independent, whereas the effects of temporal constraints depend on the nature of the motor task that is performed.