Muscle fiber conduction velocity changes with length

While recording activity from individual muscle fibers by single fiber EMG (SFEMG), stimulated either through their axons or directly, the length of the recorded muscle fiber was changed--stretched or made shorter--by manipulating the recording needle or by passive joint movements. This resulted in significant changes of latency corresponding to an increase in propagation velocity on shortening of the muscle fiber and to a slowing of its lengthening. The maximum increase in velocity was estimated to 33% and slowing to about 22%. These length-dependent changes of muscle fiber propagation velocity are suggested to contribute to the supernormal phase of propagation velocity recovery function and to be responsible for an important part of the myogenic, interdischarge interval-dependent, jitter.