Mechanically induced reflex responses in human triceps brachii.

The short and long latency reflex responses of human triceps brachii muscle were recorded in 14 healthy volunteers. An electromechanical hammer was used to stretch the muscle and recordings were made from a surface electromyogram. The monosynaptic tendon reflex occurred at a mean latency of 12.5 ms (SE 0.7 ms). Later responses were observed in activated conditions (weak force production, preparatory period) at a mean latency of 62.8 ms (SE 3.5 ms). The amplitude of the short latency reflex increased during weak tension, the long latency reflex amplitude seemed to increase during the preparatory period testing. The amplitude increases can be attributed to increased lower motoneuron excitability even during weak voluntary activity. The tendency towards an increased amplitude during the preparatory period may be connected with the higher regulation of the long latency reflex.