Enhancing Adaptations to Neuromuscular Electrical Stimulation Training Interventions

Abstract

Neuromuscular electrical stimulation (NMES) applied to skeletal muscles is an effective rehabilitation and exercise training modality. However, the relatively low muscle force and rapid muscle fatigue induced by NMES limit the stimulus provided to the neuromuscular system and subsequent adaptations. We hypothesize that adaptations to NMES will be enhanced by the use of specific stimulation protocols and adjuvant interventions.

Figure 1

A. neuromuscular electrical stimulation (NMES) may be applied via electrodes over the muscle belly or a large nerve trunk; nerve stimulation may activate all MUs within the nerve, whereas muscle belly stimulation recruits motor units (MUs) that lie closer to the stimulating electrodes; few nerves are readily accessible, so muscle belly NMES is more commonly used. B. Pulse frequency and duration as well as current intensity may be altered to achieve different outcomes. Higher frequencies and stimulation intensities both recruit more MUs and increase MU-specific force and thus evoke higher forces, but fatigue may be more rapid. Wider pulses (e.g., 1 ms) may activate sensory fibers to enhance MU recruitment through central (reflex) pathways.

Figure 2

A. Effect of neuromuscular electrical stimulation (NMES) superimposed over Achilles tendon vibration. Ankle plantar flexion torque (dark solid line) is zero at rest (section 1). Application of continuous tendon vibration (section 2) induces a tendon vibration reflex (TVR) and an increase in torque. Superposition of 2-s bursts of NMES over the muscle belly generates larger torques until a maximum response is reached (section 3). Torque remains high as vibration is continued even after NMES is ceased (section 4) and tends to remain above baseline in some individuals even after vibration is ceased (section 5). This ongoing torque, or sustained torque (T_sust), indicates a motoneuron facilitation effect, possibly resulting from activation of persistent inward currents in motoneurons. B. Changes in maximal isometric (upper panel) and isokinetic (180°·s⁻¹, lower panel) knee extension torque under NMES (open squares) and NMES + blood flow restriction (BFR, filled squares) before the training period (PRE), during the training period (MED), and immediately (POST), 1 wk (POST2), and 2 wk after the training period (POST3). Data are means ± SE. (Reprinted from 56. Copyright © 2015 American College of Sports Medicine. Used with permission.)

Figure 3

Summary of methods for increasing muscle recruitment and minimizing fatigue during, and enhancing adaptations to, neuromuscular electrical stimulation (NMES) training. Rationale for use, summary of current evidence, and indication of future potential are summarized for methods intrinsic to NMES delivery and for methods to be used concurrently with NMES. CNS, central nervous system.