Under the conditions of weightlessness, astronauts have a tendency to exhibit bone mineral loss in their spines and lower extremities (Oganov et al., 1992). According to Wolf’s law, bone adapts to functional demands by remodeling to reflect the distribution of stress placed on the bone (Bentzen et al., 1987). To prevent remodeling, NASA has suggested a plan of resistive exercises to produce the appropriate stress distributions to counteract the remodeling process. Prolonged bed rest studies are used by NASA to simulate a zero gravity environment on earth. During these bed rest studies, different exercises are performed on a device called the horizontal exercise machine (HEM). These exercises are the same type as those that are proposed for space flight.

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