The ‘late’ reflex responses to muscle stretch: the ‘resonance hypothesis’ versus the ‘long‐loop hypothesis’

Abstract

The validity of previous claims concerning the long-loop etiology of late reflex electromyogram (EMG) responses to muscle stretch in man was examined. Whether observatios previously presented in favor of the long-loop hypothesis are explicable in terms of the resonance hypothesis, according to which the late reflex components represent spinal, short-latency responses to intramuscular oscillations initiated by impact, were investigated. The contracting wrist flexors of healthy subjects were exposed to trains of recurrent 25-50 Hz stretch stimuli (wrist torque pulses). Each of the initial 2 or 3 pulses in the train was followed by EMG peaks with a latency of 20-25 ms. The EMG peaks driven in this way had the following characteristics in common with the successive 2 or 3 EMG peaks which were induced by single ramp stretches or tendon taps. Changes in stimulus parameters which altered the strength of the initial EMG peak often had an opposite effect on the strength of the succeeding peak(s). Muscle vibration which attenuated the initial peak often enchanced the succeeding one(s). The initial EMG peak was less affected than the succeeding peak(s) by the subjects'' attempts to respond with rapid resist or let-go reactions. Intramuscular oscillations (monitored by a needle accelerometer) and EMG responses evoked by single ramp stretches and/or tendon taps were also studied in the long thumb flexor, the calf muscles and the masseter muscle. In the thumb flexor, the initial accelerometer deflexion was only rarely succeeded by a short latency EMG peak, but the succeeding wave in the needle accelerogram was followed by such a peak appearing .apprx. 40 ms after stimulus application. The calf muscles and the jaw elevators exhibited a high amplitude, short-latency EMG response to the 1st but only rarely to the 2nd intramuscular oscillation wave. The interval between initial and 2nd EMG peaks following tendon taps was longer for calf muscles than for wrist flexors and longer for wrist flexors than for jaw elevators. Similar differences were observed with respect to the intervals between the damped intramuscular oscillations initiated by the impacts. Without denying the existence of long-loop reflexes, the characteristics of the late reflex responses to muscle stretch in man are explicable also in terms of the resonance hypothesis.