Muscle mechanical properties during voluntary contraction with motor-threshold neuromuscular electrical stimulation

Abstract

Abstract Neuromuscular electrical stimulation (NMES) is often superimposed on voluntary contraction, but its effects on muscle mechanical properties remain unclear. We used shear wave elastography to assess the tibialis anterior at rest and during isometric ankle dorsiflexion with and without NMES. Two experiments evaluated motor-threshold NMES ( n = 10) and perception-threshold NMES ( n = 9), with six participants completing both experiments. The shear elastic modulus was measured at 0%, 5%, and 10% of maximal voluntary contraction (MVC) under randomized crossover conditions. Motor-threshold NMES increased shear elastic modulus at 0% MVC and decreased it at 5% and 10% MVC compared with the corresponding no-NMES conditions. Perception-threshold NMES did not significantly alter shear elastic modulus. Ankle dorsiflexion force did not differ significantly between the NMES and no-NMES conditions in either experiment. An exploratory comparison of the NMES-related changes between stimulation thresholds using a linear mixed-effects model showed a significant difference at 5% MVC [F(1, 17) = 7.10, p = 0.016], but not at 10% MVC [F(1, 17) = 3.39, p = 0.083]. These findings indicate that motor-threshold NMES alters the mechanical response of the tibialis anterior during voluntary contraction.