REFLEX UNLOADING AND DEFENSIVE CAPITULATION RESPONSES IN HUMAN NECK MUSCLE

1. We studied unloading and stretch responses inhuman neck muscles dur ing manoeuvres in which the head pulled against a 2-3 kg weight which could be abruptly released or applied electromagnetically. 2. During h ead tracking in pitch, unloading of the weight induced inhibition of E MG in the contracting sternocleidomastoid at a mean latency of 24.9 ms in normal subjects and at 41 ms in bilateral labyrinthine-defective s ubjects, with antagonist (trapezius) excitation of 30.5 and 41.3 ms, r espectively. During tracking in yaw, unloading induced inhibition in t he contracting splenius capitis (SpC) at a mean latency of 20.4 ms in normal subjects and 25 ms in labyrinthine-defective subjects, with exc itation in the antagonist SpC at 22.2 and 24 ms, respectively. 3. If s ubjects tried to resist an unexpected sideways tug on the head a burst occurred in the stretched SpC at a mean latency of 53.5 ms. When subj ects relaxed there was excitation of the shortening SpC at 75.9 ms, wh ich assisted the imposed motion and is possibly a 'defensive reflex'.