Space travel "unloads" the muscles, putting them in a state of weightlessness. This means the muscles aren't working as usual, which can cause them to shrink and become weak over time.

Scientists at NASA are concerned about this problem, especially as space flights get longer. Researchers are developing special muscle training programs that can be done quickly and easily, so astronauts can maintain muscle size and strength in flight.

Doctors apply this research to prescribe just the right amount of exercise for their patients who are on bed rest or unable to use an injured limb. Muscles can stay fit without over-working.