New insights into the potentiation of the first and second phases of the M-wave after voluntary contractions in the quadriceps muscle

Introduction: We investigated the mechanisms underlying the potentiation of the first and second phases of the compound action potential (M-wave) after conditioning contractions. Methods: M-waves were evoked in the knee extensors before and after isometric maximal voluntary contractions (MVCs) of 1 s, 3 s, 6 s, 10 s, 30 s, and 60 s. The amplitude, duration, and area of the M-wave first and second phases were measured during the 10-min period after each contraction. Results: The magnitude of the M-wave first phase was enlarged only after MVCs of 30 s and 60 s, whereas the second phase increased after all MVCs, regardless of their duration. The enlargement of the first phase remained for longer than 2 min, whereas the potentiation of the second phase lasted only 20 s. Conclusions: Potentiation of the first phase is the result of fatigue-induced membrane changes, whereas enlargement of the second phase is probably related to shortening of muscle fascicles. Muscle Nerve55: 35-45, 2017