The contribution of the tendon electrode to M-wave characteristics in the biceps brachii, vastus lateralis and tibialis anterior

In some compound muscle action potentials (M waves) recorded using the belly–tendon configuration, the tendon electrode makes a noticeable contribution to the M wave. However, this finding has only been demonstrated in some hand and foot muscles. Here, we assessed the contribution of the tendon potential to the amplitude of the vastus lateralis, biceps brachii and tibialis anterior M waves, and we also examined the role of this tendon potential in the shoulder-like feature appearing in most M waves. M waves were recorded separately at the belly and tendon locations of the vastus lateralis, biceps brachii and tibialis anterior from 38 participants by placing the reference electrode at a distant (contralateral) site. The amplitude of the M waves and the latency of their peaks and shoulders were measured. In the vastus lateralis, the tendon potential was markedly smaller in amplitude (∼75%) compared to the belly M wave (P = 0.001), whereas for the biceps brachii and tibialis anterior, the tendon and belly potentials had comparable amplitudes. In the vastus lateralis, the tendon potential showed a small positive peak coinciding in latency with the shoulder of the belly–tendon M wave, whilst in the biceps brachii and tibialis anterior, the tendon potential showed a clear negative peak which coincided in latency with the shoulder. The tendon potential makes a significant contribution to the belly–tendon M waves of the biceps brachii and tibialis anterior muscles, but little contribution to the vastus lateralis M waves. The shoulder observed in the belly–tendon M wave of the vastus lateralis is caused by the belly potential, the shoulder in the biceps brachii M wave is generated by the tendon potential, whereas the shoulder in the tibialis anterior M wave is caused by both the tendon and belly potentials.