Electrophysiological characteristics of motor units and muscle fibers in trained and untrained young male subjects.

We hypothesized that the amplitudes of compound muscle action potentials (CMAPs) and interference pattern analysis (IPA) would be larger in trained subjects compared with untrained subjects, possibly due to hypertrophy of muscle fibers and/or increased central drive. Moreover, we hypothesized that the untrained muscle is less excitable compared with the trained muscle. An electromyographic (EMG) needle electrode was used to record the IPA at maximal voluntary effort. The CMAP was obtained by stimulating the musculocutaneous nerve and recording the brachial biceps muscle using surface electrodes. CMAPs were obtained by direct muscle stimulation (DMS) with two stainless-steel subdermal electrodes placed subcutaneously in the distal third of the muscle. Amplitudes of CMAP and IPA were significantly larger in trained subjects compared with untrained subjects. We found no differences between trained and untrained subjects in IPA power spectrum and turns per second or amplitude of the CMAPs obtained by DMS. Muscle fiber hypertrophy and/or altered central drive may account for our results, but there was no indication of changes in muscle fiber excitability.