Impact of different muscle lengthening amplitudes combined with electrical nerve stimulation on torque production.

This study investigated torque production resulting from the combined application wide-pulse neuromuscular electrical stimulation (NMES), delivered over the posterior tibial nerve, and muscle lengthening at two distinct amplitudes. Wide-pulse NMES (pulse duration: 1ms; 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+LEN and NMES+LEN 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+LEN (233.2 ± 101.5 Nm.s) and NMES+LEN (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+LEN (226.6 ± 115.3 Nm.s) than during NMES+LEN (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+LEN 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.