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 60s, 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 1s 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 10s, 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 these parameters, the greatest changes occurred 1s after the conditioning contraction. Changes in MFCV after the contractions were correlated with those in M-wave second phase amplitude (r²=0.42; P<0.05) and Fmedian (r²=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 muscle fiber conduction velocity

EMG Modeling | InTechOpen, Published on: 2012-10-17. Authors: Javier Rodriguez-Falces, Javier Navallas and Armando Malanda