The aim is to investigate the combined effects of hypoxia and sustained recumbency (bedrest), on human physiological systems. The partial pressure of oxygen in the environmental gas inside future planetary habitats will be lower than in atmospheric air. Prolonged exposure to low gravity will result in deconditioning of vital physiological systems, and may consequently constitute a threat to the health of the astronauts. However, it is unknown how prolonged exposure to both reduced gravity and hypoxia will affect health. The new knowledge has also implications for society in general, since chronic hypoxia and bedrest constitutes a model of the basic conditions experienced by patients suffering from respiratory insufficiency restricting them to a physically inactive life style. The challenge of the project lies in the complexity of the experimental interventions where healthy humans are confined to a hypoxic environment during prolonged bedrest. A series of studies will be conducted at the Planica hypoxia facility capable of housing 20 subjects at any simulated altitude. Subjects will remain in horizontal position (bedrest) or be ambulatory, but confined to the facility (ambulation) for 21 days/trial. Each subject will participate in three trials: hypoxic bedrest (simulated altitude 4000m), normoxic bedrest, and hypoxic ambulation. The effects will be investigated in experiments concerning metabolic, cardiorespiratory, musculoskeletal, haematological, immunological and thermoregulatory functions. In addition to the specific objectives, the study will be explorative in the sense that it will collect a broad spectrum of basic data corresponding to that obtained when 21-day bedrest experiments are conducted by ESA/NASA (bedrest core data). Thus, data from the experiments can readily be compared with core data from previous bedrest studies, and hence the added effects of hypoxia should be evident.