CHARACTERISTICS OF MOTOR UNIT DISCHARGE IN SUBJECTS WITH HEMIPARESIS

We studied the discharge rates and recruitment characteristics of sing le motor units in paretic and contralateral arm muscles of 6 hemiparet ic subjects. Motor unit activity in biceps brachii was recorded at dif ferent 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 disc harge rate of motor units in the paretic muscle. All 6 subjects showed compression of the range of motoneuron recruitment forces, and a fail ure to increase motor unit discharge rate during voluntary force incre ases in paretic muscles. These rate reductions could potentially alter the precise match of motoneuron properties to the mechanical properti es of the innervated muscle fibers, and reduce the efficiency of muscl e contraction. This reduction could lead, in turn to increased effort, to fatigue, and ultimately to a sense of weakness for voluntary force generation. (C) 1995 John Wiley & Sons, Inc.