Characteristics of motor unit discharge in subjects with hemiparesis

We studied the discharge rates recruitment characteristics of single motor units in paratic and contralateral arm muscles of 6 hemiparetic subjects. Motor unit activity in bicapes brachii was recorded at different elbow torques, and the activity related both to the mean level of surface electromyographic activity, and to the degree of weakness. In 3 of the 6 subjects, there were significant reductions in mean discharge rate of motor units in the paretic muscle. All 6 subjects showed compression of the range of motoneuron recuitment forces, and a failure to increase motor unit discharge rate during voluntary force increases in paretic muscles. These rate reductions could potentially alter the precise match of motoneuron properties to the mechanical properties of the innerved muscle fibers, and reduce the efficiency of muscle contraction. This reduction could lead, in turn to increased effort, to fatigue, and ultimately to a sense of weaknes for voluntary force generation.© 1995 John Wiley & Sons, Inc.