Impact of different muscle-lengthening amplitudes combined with electrical nerve stimulation on torque production
Résumé
This study investigated torque production resulting from the combined application of wide-pulse neuromuscular electrical stimulation (NMES), delivered over the posterior tibial nerve, and muscle lengthening at two distinct amplitudes. Wide-pulse NMES (pulse duration: 1 ms; stimulation intensity: 5-10% of maximal voluntary contraction) was delivered at both low- (20 Hz) and high- (100 Hz) stimulation frequencies, either alone (NMES condition) or combined with a muscle lengthening at two amplitudes (10 or 20° ankle joint rotation; NMES + LEN10 and NMES + LEN20 conditions, respectively). For each frequency, the torque-time integral (TTI) and the muscle activity following the cessation of stimulation trains (sustained EMG activity) were calculated. At 20 Hz, TTI was higher (P = 0.007) during NMES + LEN10 (233.2 ± 101.5 Nm·s) and NMES + LEN20 (229.2 ± 92.1 Nm·s) than during the NMES condition (187.5 ± 74.5 Nm·s), without any change in sustained EMG activity (P = 0.54). At 100 Hz, TTI was higher (P = 0.038) during NMES + LEN10 (226.6 ± 115.3 Nm·s) than during NMES + LEN20 (180.6 ± 84.0 Nm·s) and NMES (173.9 ± 94.9 Nm·s). This torque enhancement was accompanied by a higher sustained EMG activity (P = 0.045) in the NMES + LEN10 condition. These findings show that, for low-frequency NMES, significant torque increases were observed with both a 10- or a 20-degree lengthening amplitude, probably linked to increased afferents' activation. In contrast, with high-frequency NMES, a significant TTI enhancement was observed only with the 10-degree amplitude, accompanied by increased sustained EMG activity, suggesting neural mechanisms' involvement. When a greater lengthening amplitude was superimposed during high-frequency NMES, these mechanisms were probably inhibited, precluding torque enhancement.
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