EFFERENT RESPONSES TO TWITCH TESTS USED IN IDENTIFYING HUMAN MUSCLE AFFERENTS

The response of a muscle afferent to the mechanical stimulus produced by a twitch contraction of the receptor-bearing muscle is an important test for differentiating between muscle spindle endings and Golgi ten don organs. The present study demonstrates that active alpha-motor axo ns can behave in a similar manner to spindle afferents, presumably res ponding not to the mechanical event per se, but reflexly to the change in afferent discharge created by the mechanical event. alpha-Motor ax ons were unequivocally identified during microneurography using spike- triggered averages of EMG. Caution is required when the twitch test is utilized to assist in the classification of muscle afferents during a n intentional or unintentional voluntary contraction.