General Results of a 21-Day Head-Down Bed Rest Study without Use of Countermeasures

Main results of a head-down bed rest (HDBR) study performed at the Institute of Biomedical Problems are reported with an emphasis on the specifics of its organization. HDBR was used to model the physiological effects of microgravity. The study involved 12 healthy males (29.8 ± 4.6 years old, M ± SD) who volunteered for a 21-day HDBR in an antiorthostatic position (–6°). Body length, body weight, and fluid balance were assessed daily for 2 days prior to HDBR, during HDBR, and for 2 days after HDBR. Routine checkups were performed twice a day to measure the heart rate (HR), blood pressure (BP), and body temperature and to collect complaints, if any. Data analysis showed that the subjects tended to increase in body length and to lose weight during acute adaptation and that the changes persisted until the end of HDBR. Fluid balance was negative during HDBR, but became positive on completion. HR, BP, and body temperature decreased in early HDBR and increased significantly on day 1 after HDBR completion. Back discomfort, mostly in the lumbar region, was the most common complaint. The study results can be used in planning further ground-based studies with various models of the effects of lower gravity.