Effects of evoked pain on the electromyogram and compound muscle action potential of the brachial biceps muscle.

Muscle pain is often accompanied by a feeling of muscle fatigue and weakness. We examined the effect of experimental muscle pain on the electromyogram (EMG) during maximal voluntary contraction (MVC), and on the compound muscle action potential (CMAP) of the brachial biceps muscle. Twenty-one healthy subjects were injected intramuscularly with 0.1 ml/30 microg of the vanilloid receptor agonist (capsaicin) or 0.2 ml of 5% hypertonic saline. A Teflon-coated cannulated EMG needle was used to record the EMG interference pattern (IP) at MVC. The CMAP of the brachial biceps muscle was obtained by stimulation of the musculocutaneous nerve at the axilla using surface electrodes. Amplitude, mean frequency of the power spectrum, and turns/s of the interference pattern were reduced after pain induced by capsaicin or hypertonic saline. Latency, amplitude, and area-under-curve of the CMAP did not change after injection of either substance. Acute stimulation of muscle nociceptors thus produced a fatiguelike change in the interference pattern during MVC, possibly due to a decrease in motoneuron firing rate and increased muscle fatigability.