Contribution of single motor units to the surface electromyogram.

McComas (1977) has published a method of estimating the number of motor units in human sketal muscle, based on peripheral nerve stimulation and recording of the surface electromyogram (EMG). One assumption of the method is that individual motor units, recruited by peripheral nerve stimulation just above threshold, give discrete observable increments to the surface EMG which are typical of motor units in the muscle. We have investigated this assumption in the thenar muscle. An EMG needle was inserted into the muscle and a series of individual motor units, activated voluntarily, was isolated. The discharge of these units was then used to trigger averaging of the concurrent surface EMG. Our results suggest that motor units activated by peripheral nerve stimulation close to threshold are not representative, in amplitude or timing, in their contribution to the surface EMG, of all motor units in the muscle.