The Human Research Facility (HRF) is a facility at the Ames Research Center opened in 1971 where ground-based simulation studies can be carried out of the physiological responses of astronauts during spaceflight. It provides an environment with temperature, light intensity and day-length automatically controlled and is suitable for studies on both ambulatory and bed-rested volunteer subjects. Test equipment and facilities include a lower-body negative-pressure device used in studies of fluid shifts; upright and horizontal bicycles, a treadmill, and other exercise testing devices; a water-immersion tank to simulate the effects of microgravity; a man-rated centrifuge (a rotating device used to expose humans to high degrees of gravitational force) and other rotating devices; and a tilt-table for testing the body's ability to respond to an upright position after being weightless or in a head-down position for an extended time. Studies in the HRF have used healthy volunteers from various backgrounds, ranging in age from 21 to 65. For varying periods, volunteers lie in beds tilted head-down at a 6° angle. Continuous head-down bed rest is used to simulate the effects of prolonged microgravity on the human body, such as cardiovascular deconditioning, muscle atrophy, decreased bone strength, and shifts in fluid and electrolyte balance. This method of simulating the effects of weightlessness has enabled extensive study on the ground of the changes responsible for the physiological effects of spaceflight. Scientists also study exercise, diet, fluid-loading, and drugs for their effectiveness in preventing these changes. Physiological responses to a Space Shuttle reentry acceleration profile have been tested on the man-rated centrifuge. The HRF is also equipped for isolation, group interaction, human performance, and physiological rhythm studies.