Bed RestHead-Down TiltWeightlessness SimulationWeightlessness CountermeasuresRestCardiovascular DeconditioningSpace FlightBedsWeightlessnessCost of IllnessCritical IllnessOrthostatic IntoleranceSeverity of Illness IndexLower Body Negative PressureMental DisordersBed OccupancyTime FactorsIllness BehaviorMuscular AtrophyEarly AmbulationSupine PositionExerciseMuscle, SkeletalPlasma VolumeHypotension, OrthostaticHypovolemiaChronic DiseaseVascular CapacitanceImmobilizationAmino Acids, EssentialSpinal Puncture