Characteristics of the F response: a single motor unit study.

Trains of 100 to 200 stimuli result in F discharges from less than one half of motor units of hand muscles. The maximum observed F discharge frequency was 10%. There was no relation between the surface voltage of motor unit potentials and the frequency of F discharge. The motor unit potentials of larger surface voltage were recruited at higher stimulus intensity levels, usually supramaximal for the antidromic sensory nerve action potential. No correlation was observed between F latency and the surface voltage of the motor unit potentials. In only one pathological example was a clear interaction observed between the frequency of F discharge in motor unit potentials and the recruitment of other single motor unit potentials by increments in the stimulus intensity. The observations suggested that caution should be exercised before the F discharge is accepted as a method for measuring proximal conduction times in human motor nerves.