Effects of weakness on symmetrical bilateral grip force exertion in subjects with hemiparesis

It has been shown that, in a bilateral force-matching task, subjects presenting weakness in one limb produce a lower force in the weakened limb even though they subjectively perceive that they are exerting the same force. The aim of this study was to verify whether subjects with hemiparesis produced asymmetrical forces during a bilateral submaximal grip task and whether this asymmetry is related to weakness of the paretic limb. Fifteen subjects with hemiparesis and 15 healthy subjects were recruited. First, the maximal voluntary force was measured for each hand. Then, subjects were asked to exert equal forces with both hands simultaneously at three submaximal force levels using two dynamometers. In the bilateral task, the force ratios (paretic/nonparetic or nondominant/dominant) differed between groups. Severely weak hemiparetic subjects produced lower force ratios than mildly weak hemiparetic subjects and healthy subjects ( P < 0.000), whereas there was no difference between the force ratios produced by mildly weak hemiparetic subjects and those produced by healthy subjects. In subjects with hemiparesis, the force ratios in the bilateral task were related to the ratios of maximal voluntary forces (R-2 = 0.39 - 0.66, P <= 0.013) and the presence of somatosensory impairment did not affect these relationships. These results suggest that the strategy used is to compare the intensity of the motor commands on both sides and then perform the force-matching task. The use of such a strategy by subjects who have had paresis for 1 year reflects a lack of adaptation to their weakness.