Exercise microdosing for skeletal muscle health applications to spaceflight

Findings from a recent 70 day bedrest investigation suggested intermittent exercise testing in the control group may have served as a partial countermeasure for skeletal muscle size, function, and fiber-type shifts. The purpose of the current study was to investigate the metabolic and skeletal muscle molecular responses to the testing protocols. Eight males (29±2y) completed muscle power (6x4 sec; peak muscle power: 1369±86W) and VO 2 max (13±1min; 3.2±0.2L/min) tests on specially designed supine cycle ergometers during two separate trials. Blood catecholamines and lactate were measured pre, immediately post, and 4h postexercise. Muscle homogenate and muscle fiber-type specific (myosin heavy chain (MHC) I and MHC IIa) mRNA levels of exercise markers ( myostatin, IκBα, myogenin, MuRF-1, ABRA, RRAD, Fn14, PDK4) and MHC I, IIa, and IIx were measured from vastus lateralis muscle biopsies obtained pre and 4h postexercise. The muscle power test altered (p≤0.05) norepinephrine (+124%), epinephrine (+145%), lactate (+300%), and muscle homogenate mRNA ( IκBα, myogenin, MuRF-1, RRAD, Fn14). The VO 2 max test altered (p≤0.05) norepinephrine (+1394%), epinephrine (+1412%), lactate (+736%), and muscle homogenate mRNA ( myostatin, IκBα, myogenin, MuRF-1, ABRA, RRAD, Fn14, PDK4). In general, both tests influenced MHC IIa muscle fibers more than MHC I with respect to the number of genes that responded and the magnitude of response. Both tests also influenced MHC mRNA expression in a muscle fiber-type specific manner. These findings provide unique insights into the adaptive response of skeletal muscle to small doses of exercise and could help shape exercise countermeasures for astronauts and Earth-based populations (e.g., aging individuals).