Neuromuscular fatigue induced by the combined application of neuromuscular electrical stimulation and muscle lengthening

This study compared neuromuscular fatigue induced by an acute wide-pulse high-frequency session, either applied in an isometric condition (WPHF) or combined with muscle lengthening (WPHF + LEN). Fifteen participants completed two randomized sessions, which involved 30 stimulation trains (pulse duration: 1 ms; frequency: 100 Hz; duty cycle: 15 s ON/15 s OFF) applied to the posterior tibial nerve at low stimulation intensity [5%-10% maximal voluntary contraction (MVC)]. In the WPHF session, the ankle joint was held at a reference angle (90°), whereas a 10° muscle lengthening was superimposed during the stimulation in the WPHF + LEN session. Before and after each session, MVC was measured along with neural [voluntary activation level (VAL)] and muscular [potentiated twitch (Pt)] changes. Torque-time integral (TTI) was recorded for each train, and the total TTI (∑TTI) was calculated. Results showed a comparable decrease in MVC torque after the two sessions (-7.8 ± 6.9% for WPHF and -9.4 ± 5.7% for WPHF + LEN, <i>P</i> < 0.001) associated with a significant reduction in Pt amplitude (<i>P</i> < 0.001), indicating muscular changes, whereas VAL remained unchanged. ∑TTI was not different between sessions (9,600 Nm·s for WPHF; 9,550 Nm·s for WPHF + LEN; <i>P</i> = 0.95). However, although TTI significantly decreased throughout the WPHF session, it was preserved during the WPHF + LEN session. These findings indicate a similar amount of neuromuscular fatigue after the two sessions, primarily attributed to muscular alterations. Nevertheless, the combination of WPHF stimulation with muscle lengthening appears advantageous for preserving torque production throughout the stimulation trains.<b>NEW & NOTEWORTHY</b> Results of the present study indicate that a single session of WPHF stimulation modality either applied alone or with muscle lengthening induces the same level of neuromuscular fatigue. Despite no significant difference in total evoked torque between sessions, the present findings highlight potential advantages of superimposing muscle lengthening to preserve torque production during repeated WPHF trains.