H-reflexes in the upper extremity and the effects of voluntary contraction.

We report a simple technique for obtaining H-reflexes from the extensor carpi radialis (ECR) muscle. In 50 normal test subjects, above-elbow stimulation of the radial nerve evoked ECR H-reflexes, in all 100 arms with facilitation, and in only 8 without facilitation. H-waves were less common in extensor digitorum communis and rare in brachioradialis; these could have been volume-conducted ECR responses. Median-nerve stimulation at the elbow evoked H-reflexes in 92 of the 100 flexor carpi radialis (FCR) muscles; 44 of these required no facilitation. Background EMG (facilitation) activity or an overlapping M-wave often obscured the H-wave onset: among the onsets considered reliable in FCR (67/92) and ECR (84/100), mean latencies were 16.2 msec and 16.5 msec respectively, and correlated well with subject height and arm length. There was no significant difference in latency obtained with and without facilitation for the FCR H-reflexes and between techniques for the ECR H-reflexes. Right-left differences in latency ranged from 0 to 0.9 msec for ECR and 0 to 0.6 msec for FCR H-reflexes.