Initial motor unit recruitment in patients with spastic hemiplegia.

Lesions of the first motor neuron provoke abnormal voluntary movements. To clarify the central nervous system mechanisms underlying these changes we analyzed the behavior of the first motor unit recruited during a minimal effort tonic contraction of the deltoid and abductor digiti minimi manus muscles in patients with hemiplegia due to cerebrovascular lesions in the distribution of the middle cerebral artery. We compared data from paretic and healthy muscles in the same subject. The onset and recruitment intervals determined for the single motor unit yielded the range of control. The first recruited motor unit had a lower baseline firing rate and the second recruited motor unit potential appeared significantly earlier (p < 0.01) in plegic than in healthy muscles. Both changes affected distal more than proximal locations. Recordings from plegic muscles, particularly at distal locations, also disclosed a lowered range of control. These findings suggest that in hemiplegic patients the central nervous system loses its ability to modulate the frequency of firing during minimal effort voluntary movements so that distal muscles tend to behave like proximal muscles.