The effect of weightlessness on humans results in a loss of muscle strength and volume. Similar to bone deterioration, skeletal muscles atrophy as a result of disuse. In space, actions and movement require considerably less exertion because the force of gravity is practically nonexistent. As a result, astronauts’ skeletal muscles become deconditioned.

The effects of weightlessness on cardiac (heart) muscles resembles what happens to skeletal muscles in that a weakening can result under such conditions. Just as an athlete will strengthen his or her heart muscles through exercise and make them more efficient as a pump, any reduction in demand will lessen this efficiency.