Objective: To investigate waveform changes of compound muscle action potentials (CMAPs) related to voluntary muscle contraction and alteration of muscle length and to evaluate the effect of peripheral neuropathy on temporal and spatial summations of CMAPs.

Method: The influence of voluntary muscle contraction and alteration of muscle length on CMAP was studied in 37 median nerves of 21 patients with median neuropathy.

Results: In patients with no apparent axonopathy, temporal summation was partially disturbed without significant change of spatial summation. Shortening of muscle length or voluntary contraction produced a physiologic improvement of spatial and temporal summations. There was a decrease in temporal and spatial summations, more prominent in temporal summation, with lengthening of the muscle. In axonopathy, spatial summation was markedly deteriorated with partial reduction of temporal summation. Spatial summation was not affected by the change of muscle length or voluntary contraction. Temporal summation was improved by muscle shortening or voluntary contraction and was decreased by muscle lengthening.

Conclusion: Peripheral neuropathy has an effects on physiological spatial and temporal summations of CMAPs. Temporal summation is preferentially decreased in cases without axonopathy. When axonopathy is apparent, spatial summation is profoundly disturbed with partial reduction of temporal summation.

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.