Muscle Fatigue during Neuromuscular Electrical Stimulation is Dependent on Training: A Basis for Microgravity Musculoskeletal Countermeasure Design
Thomas Joseph Abitante
HST, PhD Candidate
The muscle contraction creates large internal forces and can be used to provide strain on the bone to attenuate bone loss during spaceflight, which will be necessary in order to reduce risk of injury on a future long duration spaceflight missions to the Moon or Mars, where a robust exercise regimen will not be feasible. To aid in developing a Neuromuscular Electrical Stimulation (NMES) regimen for use as a spaceflight musculoskeletal countermeasure, the fatigability of the quadriceps muscles of competitive athletes of different training modalities (endurance, explosive, and control) during long duration, repetitive NMES against a fixed resistance (NMES-FR) was accessed. NMES-FR was administered to the vastus medialis for 20 minutes, at a duty cycle of 1 second on, 3 seconds off, resulting in 300 total isometric contractions, and the isometric knee extension force output of each contraction was recorded with an externally fixed dynamometer. The endurance group fatigued significantly less than both the explosive and control groups in all measured fatigue metrics (p <0.05). This study demonstrated that in the absence of high intensity endurance training, healthy individuals will fatigue rapidly during repetitive NMES. Therefore, during a NMES treatment targeting bone, regimens consisting of repetitive contractions should be short, and distributed throughout the day in order to maintain a maximum muscle contractile force.
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