The distribution of muscular weakness in upper motor neuron lesions affecting the arm.

The strength of 12 muscle groups of the arm was measured to determine the distribution of upper motor neuron weakness in man. Three groups of subjects were studied: 14 intact volunteers (both sides recorded), 10 patients with unilateral arm paresis (both sides recorded) and 6 patients with severe paralysis of the arm (the 'intact' arm only measured). On the side contralateral to the causative lesion the pattern of weakness was not the same in all patients. Shoulder muscles were relatively spared while the wrist and finger flexors were relatively severely affected, the difference being statistically significant. In hemiparetic and hemiplegic patients the strength of muscles ipsilateral to the lesion was reduced compared with normal controls. These observations refine previous clinical descriptions of upper motor neuron weakness and have implications for its pathophysiology.