During the first 11-15 days of the study participants will be free to move around inside the bed rest facility and do normal things. They will also take part in a number of tests to find out the normal state of their bone, muscle, heart and circulatory system, brain and nervous system, and vestibular (inner ear balance) system as well as their nutritional condition and their ability to fight off infections.

After the first 11-15-day period, participants will spend 90 days lying in bed, (except for limited times for specific tests) with their body slightly tilted downward (head down, feet up). Every day, they will be awake for 16 hours and lights out (asleep) for 8 hours. During the bed rest time they will also take part in a number of tests to find out changes in the state of their bone, muscle, heart and circulatory system, and nervous system, as well as their nutritional condition and their ability to fight off infections.

During the final 14 days of the study (called the recovery period), participants will again be free to move about within the facility. Because of deconditioning that takes place during bed rest, they will slowly begin normal everyday activity. During the recovery period, they will again take part in tests of their bone, muscle, heart and circulatory systems, and nervous systems, as well as their nutritional condition and their ability to fight off infections.

Participants must be nonsmokers who are in good health with no history of cardiovascular, neurological, gastrointestinal, or musculoskeletal problems.

Individuals selected for screening will receive a physical examination at no cost. Once medically qualified, subjects will be compensated for their time spent for the required psychological assessment and any additional screening.