Agonist and antagonist EMG activation during isometric torque development at the elbow in spastic hemiparesis

Voluntary isometric step contractions of the elbow flexor and extensor muscles were studied in a group of patients with paresis arising as the result of unilateral cerebral lesion and in a control group of normal subjects. For each subject the maximum isometric torque in flexion and extension was obtained, along with a series of graduated torque steps up to this maximum, in order to perform a regression analysis between torque developed and the associated agonist and antagonist EMG. This relationship proved to be linear in all normal subjects and in all but the most paretic spastic patients. If the patients were grouped according to their ability to make discrete large angle flexion and extension movements at the elbow, a clear correspondence was seen between increasing movement disability and the degree of paresis. No significant differences were found in the torque/EMG relationship of spastic patients when either elbow extensors or flexors were acting as the agonist in a contraction. Similarly, no evidence of exaggerated antagonist co-activation was found. It is concluded that, in the upper arm muscles, hemiparesis following stroke cannot, under isometric conditions, be attributed to hyperactivity of antagonist muscles.