1. When a muscle is unloaded during voluntary contraction, there is normally a silent period in the electromyogram. The silence is terminated by a sudden return of muscle action potentials.2. In order to investigate the mechanism of the terminal motor volley, the unloading reflex was studied in six human subjects. The independent variables were the initial muscular force, the inertia of the limb and the amount of motion permitted. The dependent variables were the size and latency of the terminal volley.3. During isometric contraction, the amplitude of the surface-recorded muscle action potentials increased monotonically with increasing muscular tension.4. The action potentials were significantly larger during the terminal volley than during the period before unloading.5. When acceleration of the limb was reduced by increasing the inertia, the terminal volley was decreased in size, but the latency was not affected.6. When movement was interrupted by a mechanical block, the latency of the terminal volley was reduced, but the size was not affected.7. The results suggest that the terminal motor volley is not the result of a decrease in Renshaw feed-back or in autogenetic inhibition.8. The motor volley must be regulated by proprioceptive feed-back, because it is affected by the velocity and displacement of the limb.9. The muscle frequently responded within 20 msec after motion of the limb was blocked. Hence it appears that the mechanism involves a spinal reflex.10. Because the motor discharge occurs while the muscle is shortening, it cannot be an ordinary stretch reflex. If the discharge is attributed to spindle afferent driving, one must assume that the gamma motor neurones are active during the silent period.11. The authors postulate a fusimotor reflex, which is driven by afferent impulses from the moving limb and excites the alpha motoneurones by way of the `gamma loop'.
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