Extraneous movement associated with hemiplegic postural sway during dynamic goal-directed weight redistribution

The locus of postural stability during dynamic, visually cued weight redistribution was studied in six hemiplegic and five able-bodied subjects. After a verbal warning with a variable foreperiod, a visual trace appeared on an oscilloscope to indicate a sway magnitude equaling half the subject's maximal voluntary sway. Body movement was cued in three directions--forward, and toward the paretic and nonparetic lower extremities. In order to assess extraneous postural patterns, all three directions of sway were analyzed for each trial. The results showed that with cued lateral sway (right or left) there was a tendency for hemiplegic subjects to reduce the dispersion (or variability) of the sway path. Clustering of the active lateral postural response near the geometric center of the sway path indicated a restricted balance response in this plane of motion. During cued forward sway the variability of the sway path was not statistically different from the able-bodied, but three-dimensional visualization of the sway pattern illustrated a qualitatively different postural response with extraneous, nonfocused balance activity biased toward the nonparetic lower extremity. Short-term practice did not alter the dispersion of the sway path for hemiplegic or able-bodied subjects in any direction. Hemiplegic extraneous movement in one case (forward sway) and restricted movement in another (lateral sway) emphasize the level of specificity which needs to be addressed during rehabilitation of balance.