Neuromuscular electrical stimulation (NMES) is commonly used in clinical settings to evoke contractions in an effort to enhance the rehabilitation of human skeletal muscles.1,2 This chapter will (1) review early training studies that incorporated NMES training of skeletal muscle, (2) provide data to support the idea that properly designed NMES training protocols can result in improved neuromuscular performance, (3) provide guidelines for the implementation of NMES training for individuals with musculoskeletal disorders or other conditions, (4) provide an overview of NMES-induced muscle recruitment, and (5) outline some of the drawbacks associated with NMES training. This information is intended to assist practitioners by providing evidence-based treatment when utilizing NMES to improve muscle size and strength.
RATIONALE FOR NEUROMUSCULAR ELECTRICAL STIMULATION
The goal of using NMES is to elicit contraction of skeletal muscle to obtain the physiological improvements that result from exercise training. NMES is a widely accepted modality that is used to treat atrophic muscle after injury or disease, although a consensus on specific programs and parameters to improve neuromuscular performance has yet to be reached. Early studies targeting increases in muscle mass were not consistently successful. However, recent studies have demonstrated the potential of NMES training protocols to elicit a hypertrophic response in skeletal muscle, suggesting that this training modality has the potential to be a valuable tool for rehabilitation when utilized appropriately.
The neuromuscular system is perhaps the most highly plastic system in the human body, showing dramatic adaptations in response to changes in activity level. Skeletal muscle can adapt by increasing or decreasing the amount of contractile proteins, by changing its fiber type composition, or by altering its metabolic profile to sustain force production.3,4 Adaptations resulting from activity are not limited to peripheral skeletal muscle, with concurrent adaptations occurring in neural systems in response to activity.5-7 Specifically, enhanced recruitment of motor units (an alpha motor neuron and all the muscle fibers innervated by it) is accomplished through improved central control of firing frequency or synchronization of motor units—that is, by activating motor units more frequently and in unison, muscle force can be increased. These factors underlie the use of NMES.
NMES FOR MUSCLE STRENGTHENING
NMES is used clinically as a tool to strengthen weakened muscle in persons with musculoskeletal disorders and is supported by evidence in the scientific literature. Muscle strengthening is thought to result from two primary mechanisms: increased muscle size or improved motor unit recruitment (neural adaptations).8 Increasing muscle mass usually takes several weeks to occur, while neural adaptations can occur more rapidly. The neural adaptations are typically due to increased motor unit recruitment, which is caused by (1) increasing the number of motor units recruited, (2) increasing the frequency that motor units are recruited (Fig. 13-1), or (3) recruiting motor units in a more synchronized manner (i.e., at the same time)....