Bed RestHead-Down TiltWeightlessness SimulationWeightlessness CountermeasuresRestCardiovascular DeconditioningSpace FlightBedsWeightlessnessOrthostatic IntoleranceLower Body Negative PressurePhysiciansMedical Staff, HospitalBed OccupancyPhysician-Patient RelationsSupine PositionMuscular AtrophyTime FactorsEarly AmbulationMuscle, SkeletalPlasma VolumeExerciseHypotension, OrthostaticAttitude of Health PersonnelHypovolemiaFamily PracticeVascular CapacitanceAmino Acids, EssentialImmobilization