The 'late' electromyographic response to limb displacement in man. II. Sensory origin.

Results from a previous article suggested that long-loop pathways probably contribute to the ‘late’ EMG response to sustained muscle stretch (FSR) in man. The present experiments were undertaken to investigate the relative contribution by muscle and other afferents to this response particularly in view of the fact that cutaneous stimulation has been shown to elicit long-loop spino-bulbo-spinal reflexes in man. Our results showed no significant change in response latency of the FSR recorded electromyographically from the gastrocnemius to a suddenly applied and maintained dorsiflexing force before, during and after full recovery from complete regional anesthesia of the ankle and foot. The respective mean latencies were 99 (± S.E. 4.6), 98 (± S.E. 4.8) and 99 (± S.E. 4.1) msec and were therefore indistinguishable from one another as were their response amplitudes. Thus, these experiments provide evidence against contribution to the early phase of the FSR by the cutaneous and joint afferent volleys arising from the ankle and foot. The FSR to a sustained stretching force was then compared with the voluntary EMG activity evoked in the gastrocnemius following a similar forceful stimulus but which produced negligible ankle dorsiflexion and hence negligible muscle stretch. The mean latencies were respectively 107.3 (± S.E. 1.6) and 131.4 (± S.E. 2.2) msec which represent a highly significant difference of means (P < 0.001). The auditory cue emitted by the stretching force applied to the foot was also found to evoke a voluntary EMG activity with a significantly longer latency (mean = 138.1 ± S.E. 3.3 msec) than the FSR. Hence it is inferred that in these experiments, at least the early component of the FSR originated predominantly from muscle afferents stimulated by muscle stretch. The flexibility of such a myotatic response system in man is discussed.