Bed RestDeuteriumAir PollutantsHead-Down TiltKineticsWeightlessness SimulationWeightlessness CountermeasuresMagnetic Resonance SpectroscopyRestCardiovascular DeconditioningSpace FlightBedsWeightlessnessOrthostatic IntoleranceLower Body Negative PressureBed OccupancyTime FactorsSupine PositionMuscular AtrophyMuscle, SkeletalExercisePlasma VolumeEarly AmbulationHypotension, OrthostaticHypovolemiaVascular CapacitanceAmino Acids, EssentialImmobilizationTreatment Outcome