Key result
Conditioning contractions significantly increased the amplitude of the second phase of the M wave, which correlated with increased muscle fiber conduction velocity (r2=0.42; P<0.05).
Potentiation of the second phase of the M wave after voluntary contractions is mainly due to increased muscle fiber conduction velocity.
May refine M-wave interpretation during neuromuscular testing; leaves open its clinical diagnostic utility.
The study was undertaken to provide insight into the mechanisms underlying the potentiation of the muscle compound action potential (M wave) after conditioning contractions. M waves were evoked in the tibialis anterior before and after isometric maximal voluntary contractions (MVC) of 1, 3, 6, 10, 30, and 60 s, and after 3-s contractions at 10, 30, 50, 70, 90, and 100% MVC. The amplitude, duration, and area of the first and second phases of the M wave, together with the median frequency (Fmedian) and muscle fiber conduction velocity (MFCV) were recorded. Furthermore, twitch force, muscle fascicle length, and pennation angle were measured at rest, before, and 1 s after the conditioning contractions. The results indicate that only the amplitude of the second phase of the M wave was significantly increased after conditioning contractions. The extent of this potentiation was similar for MVC durations ranging from 1 to 10 s and augmented progressively with contraction intensity from 30 to 70% MVC. After these conditioning contractions, the duration and area of the two M-wave phases decreased (P < 0.05), whereas MFCV and Fmedian increased (P < 0.05). For all of these parameters, the greatest changes occurred 1 s after the conditioning contraction. Changes in MFCV after the contractions were correlated with those in M-wave second-phase amplitude (r(2) = 0.42; P < 0.05) and Fmedian (r(2) = 0.53; P < 0.05). In contrast, fascicle length and pennation angle did not change after the conditioning contractions. It is concluded that the potentiation of the second phase of the M wave is mainly due to an increased MFCV.
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Rodríguez-Falces et al. (2015) studied this question. Isometric voluntary contractions vs. Rest/before conditioning contractions was evaluated on Amplitude of the second phase of the M wave (p=<0.05). Conditioning contractions significantly increased the amplitude of the second phase of the M wave, which correlated with increased muscle fiber conduction velocity (r2=0.42; P<0.05).
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