Neuromuscular Electrical Stimulation to Combat Muscle Atrophy During Spaceflight: A Narrative Review of Mechanisms and Potential Applications

As humanity continues to strive for extraplanetary exploration, which is quickly gaining marked governmental and industrial support and recognition, there are still substantial detriments to astronaut health during long-duration spaceflight (i.e., muscle atrophy) that must be addressed. The effects of long-duration spaceflight on muscle architecture, morphology, and function have been well documented since the Apollo and Space Shuttle Programs. Countermeasures focused on resistance or aerobic training, such as the Advanced Resistive Exercise Device, Multi-modal Exercise Device, flywheel exercise, and aerobic exercise on a mounted treadmill and/or a cycle ergometer with vibration isolation system, have been assessed to combat the functional and mechanical losses in muscle while astronauts are in low Earth orbit. However, a lesser-understood countermeasure to muscle atrophy during spaceflight is neuromuscular electrical muscle stimulation (NMES). Although utilization in spaceflight is limited, ground-based research on NMES in diseased or injured populations demonstrates its effectiveness as a promoter of muscle anabolism and growth. The previous literature has suggested the use of electrical muscle stimulation as a low-effort modality of exercise for astronauts, which could effectively enhance astronaut health and contribute to mission success. The efficacy and mechanisms of action of using NMES to attenuate atrophy in astronauts will be discussed in this review.