SKELETAL MUSCLES ARE DYNAMIC MECHANICAL AND METABOLIC MACHINES THAT DRIVE BODY MOVEMENT AND ENERGY EXPENDITURE. SKELETAL MUSCLES ARE DYNAMIC TISSUES THAT CAN ADAPT QUICKLY TO ALTERATIONS IN MECHANICAL UNLOADING BY ALTERING THEIR MASS AND MUSCLE FIBER CROSS-SECTIONAL AREA. SKELETAL MUSCLE STRENGTH AND ENDURANCE ARE ESSENTIAL TO THE HEALTH WELL-BEING AND PERFORMANCE OF ASTRONAUTS DURING SPACEFLIGHT AND UPON RETURN TO A GRAVITATIONAL ENVIRONMENT. THE MECHANICAL UNLOADING DUE TO THE MICROGRAVITY (.G) OF SPACEFLIGHT CAUSES MUSCLE FIBER ATROPHY AND FIBER-TYPE SHIFT OF POSTURAL MUSCLES IN THE LOWER EXTREMITIES AND FLEXORS IN THE UPPER EXTREMITIES. MICROGRAVITY ALSO INCREASES THE RISK OF SKELETAL MUSCLE DAMAGE WEAKNESS AND THUS THE RISK OF INJURY UPON RELOADING (E.G. EVAS MARS).