The influence of voluntary muscle contraction and alteration of muscle length on compound muscle action potential (CMAP) was studied in 20 healthy volunteers. The CMAPs were evoked by a supramaximal stimulation and recorded by a surface electrode array. Onset latencies were not significantly changed regardless of the muscle length and contraction. On shortening of the muscle there was a decrease in an area and duration of CMAP with no significant changes in an amplitude. During muscle contractions, there was an increase in amplitude and a decrease in area and duration. On a lengthening of the muscle with relaxation, the amplitude of CMAP decreased with an increased area and duration. During a voluntary contraction, there was a decrease in amplitude, area, and duration. The results are considered due to a peripheral factor such as an alteration of temporal or spatial summation rather than a central mechanism. We conclude that in nerve conduction studies, it is important to monitor the finger position and muscle relaxation to differentiate the waveform changes from the muscle length or contraction and those from the nerve lesions.