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

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 (<i>P</i> = 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 (<i>P</i> = 0.54). At 100 Hz, TTI was higher (<i>P</i> = 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 (<i>P</i> = 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.<b>NEW & NOTEWORTHY</b> This study demonstrates that combining wide-pulse low-frequency NMES and muscle lengthening can increase torque production compared with the sole application of NMES. Torque enhancement is most likely linked to the persistent firing of muscle afferents. Although muscle lengthening superimposition also permitted torque increases during wide-pulse high-frequency NMES, increasing the muscle lengthening amplitude did not allow further torque enhancements, probably due to presynaptic inhibitory mechanisms.