If you are selected to participate in the study, you will be scheduled to spend about 41 days living in the research unit called the General Clinical Research Center (GCRC) at the University of Texas Medical Branch in Galveston, TX. During the first 11 days of the study you will be free to move around inside the bed rest facility and do normal things. You will also take part in a number of tests to find out the normal state of your bone, muscle, heart, circulation, and nervous systems, as well as your nutrition and immune status. After the first 11-day period you will spend 21 days confined to strict bed rest, (except for limited times for specific tests or treatments). You will be in bed with your body tilted downward by six degrees (head down, feet up). During this time you will also take part in a number of tests to find out changes in the state of your bone, muscle, heart, circulation, and nervous systems, as well as your nutrition and immune status. If you are assigned to the treatment group, you will spin on the human centrifuge for 1 hour/day during this period. If you are assigned to the control group, you will be placed on the centrifuge for 1 hour/day but you will not have daily spins. During the final 9 days of the study (called the recovery period), you will again be free to move about within the facility, but you cannot leave. Because of the deconditioning that takes place during bed rest, you will slowly begin normal everyday physical activity.

Participants must be nonsmokers between the ages of 25-55 years old, who are in good health with no history of cardiovascular, neurological, gastrointestinal, or musculoskeletal problems. Individuals susceptible to motion sickness may be excluded from participating.

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.