The objective of this study is to investigate whether a cycling exercise coupled with artificial gravity via a short-arm human centrifuge helps to prevent and / or reduce the deleterious effects induced by 60 days of anti-orthostatic bedrest. The secondary objective is to investigate whether the combination of a supine cycling exercise with artificial gravity is more effective than the same supine cycling exercise alone in preventing or reducing the effects of head-down bedrest. During a randomized, 60 day bed rest study, in 24 healthy male adults, the two following aims will be undertaken: * Fourteen scientific protocols will assess the changes in the cardiovascular, metabolic, musculoskeletal, neuro-sensorial, haematological, and immunological systems. * In the above-mentioned systems, the comparative potential beneficial effects of the two countermeasure protocols will also be investigated by the scientific protocols and bedrest standard measurements (BSM).

Space flights have shown the possibilities and limitations of human adaptation to space. For the last 60 years, results have shown that the space environment and microgravity in particular, cause changes that may affect the performance of astronauts. These physiological changes are now better known: prolonged exposure to weightlessness can lead to significant loss of bone and muscle mass, strength, cardiovascular and sensory-motor deconditioning, immune, hormonal and metabolic changes . Moreover, recently a new suite of physiological adaptations and consequences of space flight has been acknowledged. Indeed, after long flights, some astronauts present persistent ophthalmologic changes, mostly a hyperopic shift, an increase in optic nerve sheath diameter and occasionally a papillary oedema now defined by National Aeronautics and Space Administration (NASA) as Spaceflight-Associated Neuro-ocular Syndrome (SANS). Some of these vision changes remain unresolved for years post-flight. This phenomenon has most likely existed since the beginning of human space flight but is just recently being recognized as a major consequence of adaptation to microgravity. Overall, spaceflight induces physiological multi-system deconditioning which may impact astronauts' efficiency and create difficulties upon their return to normal gravity. Understanding the underlying mechanisms of these processes and developing efficient countermeasures to prevent, limit or reverse this deconditioning remain important challenges and major priorities for manned space programs. The space agencies are actively engaged in studying the physiological adaptation to space environment through studies on board the International Space Station (ISS) but also on the ground. Indeed, considering the limited number of flight opportunities, the difficulties related to the performance of in-flight experiments (operational constraints for astronauts, limited capabilities of in-flight biomedical devices), ground-based experiments simulating the effects of weightlessness are used to better understand the mechanisms of physiological adaptation, design and validate the countermeasures. Different methods are used to simulate microgravity on Earth. However, two approaches, -6° head-down bed rest (HDBR) and dry immersion (DI) have provided possibilities for long-term exposures with findings closest to those seen with a weightless state. They produce changes in body composition (including body fluid redistribution), cardiovascular and skeletal muscle characteristics that resemble the effects of microgravity. One of the advantages of the HDBR model is that it has now been used in a great number of studies internationally, and its effects have long been described and compared with those of microgravity and spaceflight. Long-term bedrest is the gold-standard method for studying the effects of weightlessness and to test countermeasures. The HDBR, as the name implies, implicates a long (from several weeks to a year) stay in the supine position, the head tilted down by -6° from the horizontal plane. HDBR is the most frequently used ground-based simulation for gravitational unloading of the human body in western countries. During human space missions, the current most effective countermeasure is physical exercise. However, it is both time-consuming and not completely satisfactory. One of the solutions for this is to combine physical exercise with artificial gravity, with the use of a short-arm human centrifuge (SAHC). This study proposes to test the effectiveness of a countermeasure protocol combining Artificial Gravity (AG) with a cycling exercise, and to compare it with only a cycling exercise, and with a complete lack of physical exercise.