Long-latency stretch reflex induced by tendon jerk in human triceps brachii muscle

The electrical activity of the human triceps brachii muscle was recorded with surface electrodes in 14 healthy volunteers. Three experimental conditions were used to analyze muscle responses produced by tendon tap with an electromechanical hammer: 1) the muscle was at rest, 2) the subjects maintained an isometric tension for constant background EMG activity and 3) the subjects were asked to extend their arm in response to an acoustic signal occurring simultaneously with the tap. The analyses were performed with a computer. The monosynaptic tendon reflex had a mean latency of 12,5 ms. Later responses were observed with a mean latency of 62.8 ms. The later response observed in the extension experiments showed a synchronous EMG activity in comparison to the voluntary activity. It seems likely that proprioceptive impulses are of major importance in producing also the late responses since the method used minimalized cutaneous and joint afferent inputs.