Influence of sustained stretch on late muscular responses to magnetic brain stimulation in patients with upper motor neuron lesions.

Late muscular responses to transcranial magnetic stimulation occur in healthy subjects only in tonically active muscles with a proportional relationship between the amount of the EMG-response and the voluntary innervation strength. In hemiparetic stroke patients late potentials are elicited in chronically spastic hand flexors without voluntary background muscular activity probably reflecting enhanced excitability of spinal alpha-motoneurons in the spastic state. When spastic muscle hypertonus has been diminished by sustained muscle stretch late EMG-potentials are reduced or have disappeared completely. The relation between spastic muscle hypertonus and the late muscular response potentials as well as the possible mechanisms of sustained muscle stretch on the response characteristics to transcranial magnetic stimulation in spastic flexor muscles of the hand are discussed.