Dynamic response of human muscle spindle afferents to stretch during voluntary contraction

1. The response of twenty-eight human muscle spindle afferents from m. exte nsor carpi radialis brevis to large amplitude ramp stretch and release at t he wrist joint was recorded. The dynamic index was calculated as the differ ence in firing rate between that just before the end of stretch and that du ring the subsequent static phase of stretch. The value during steady volunt ary contraction was compared with that during relaxation. 2. In twenty-three primary afferents, the dynamic index increased in eleven and decreased in twelve afferents with a range of -8 to +25 impulses s(-1) . In five secondary afferents the change was less than 2 impulses s(-1). 3. The primary afferents abruptly stopped firing when the stretch was relea sed in the relaxed muscle. This cessation was prevented during contraction in seventeen primary afferents. 4. The results suggest the presence of dynamic and static fusimotor actions on the human muscle spindles during voluntary contraction.