Characteristics of EMG responses to imposed limb displacement in patients with vascular hemiplegia

The segmented EMG activity in flexor carpi radialis to imposed wrist movements was studied in 18 hemiplegic subjects with vascular lesions of the sensorimotor cortex or internal capsule and compared to that in age-matched normal subjects. The segmented EMG activity in the stretched muscle was normalized against the maximum M response elicited by electrical stimulation of its peripheral nerve in hemiplegic and normal subjects. This M response was used to estimate the maximal activity of the motoneuron pool. Twelve of the hemiplegic patients showed a stereotyped abnormal segmentation pattern of the EMG activity characterized by: an increased M1 segment of prolonged duration with both an increased sensitivity to low initial velocities of displacement and an increased slope of the input-output relationship for the range of velocities tested; absent or diminished activity during the interval of the normal M2-3 segment; and a late component not evident in normal subjects. The increased magnitude of the EMG activity during the M1 interval, markedly exceeded the normal range of control values. A method to demonstrate the response distribution of the interrelationship of the magnitude of the M1 segment using two variables (background EMG activity and initial velocity) for individual responses rather than average responses is presented. The results of the study indicate that: the increased excitability, reflected in the enhanced EMG over the M1 segment does not result from increased levels of resting alpha motoneuron activity and may result from disruption of polysynaptic internuncial influences on alpha motoneurons; the absent or diminished activity over the interval of the normal M2-3 segment may partially result from the interruption of a transcortical reflex by the vascular lesions.