<P> Inactivity and starvation in mammals lead to atrophy of skeletal muscle, a decrease in muscle mass that may be accompanied by a smaller number and size of the muscle cells as well as lower protein content . Muscle atrophy may also result from the natural aging process or from disease . </P> <P> In humans, prolonged periods of immobilization, as in the cases of bed rest or astronauts flying in space, are known to result in muscle weakening and atrophy . Atrophy is of particular interest to the manned spaceflight community, because the weightlessness experienced in spaceflight results is a loss of as much as 30% of mass in some muscles . Such consequences are also noted in small hibernating mammals like the golden - mantled ground squirrels and brown bats . </P> <P> During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass, known as sarcopenia . The exact cause of sarcopenia is unknown, but it may be due to a combination of the gradual failure in the "satellite cells" that help to regenerate skeletal muscle fibers, and a decrease in sensitivity to or the availability of critical secreted growth factors that are necessary to maintain muscle mass and satellite cell survival . Sarcopenia is a normal aspect of aging, and is not actually a disease state yet can be linked to many injuries in the elderly population as well as decreasing quality of life . </P> <P> There are also many diseases and conditions that cause muscle atrophy . Examples include cancer and AIDS, which induce a body wasting syndrome called cachexia . Other syndromes or conditions that can induce skeletal muscle atrophy are congestive heart disease and some diseases of the liver . </P>

3 main sources of atp available for human muscle cells