Reflex connections from forearm and hand afferents to shoulder girdle muscles in humans.

Using surface electromyographic recordings from the trapezius and serratus anterior muscles and percutaneous electrical stimulation of the median, ulnar and radial nerves, the reflex connections from forearm and hand afferents to these shoulder girdle muscles have been investigated in normal human subjects. Stimulation of the median, ulnar and radial nerves at the shoulder, elbow and wrist evoked late, excitatory reflexes in the upper and lower parts of trapezius and in serratus anterior. These reflexes are not evoked by stimulation of cutaneous afferents alone, since there was no response to stimulation of the distal cutaneous branches of these three nerves. Measurements of the conduction velocity of afferents of the median, ulnar and radial nerve evoking these reflexes gave a mean conduction velocity of approximately 50 m/s. The lowest stimulus intensities at which these reflexes could be evoked were found to be 0.3 times motor threshold (MT). By taking into account the reflex latency, the length of the conduction path and that the reflex was mediated by low threshold, fast conducting afferents, it is proposed that group I muscle afferents from the forearm or hand evoke a supraspinal reflex to trapezius and serratus anterior. It appears that the functional significance of these reflexes is to aid in the stability of the shoulder girdle.