Bed RestHead-Down TiltNo-Observed-Adverse-Effect LevelWeightlessness SimulationWeightlessness CountermeasuresRestCardiovascular DeconditioningSpace FlightBody WeightBedsTime FactorsOrgan SizeWeightlessnessDouble-Blind MethodFecesDose-Response Relationship, DrugDietOrthostatic IntoleranceLower Body Negative PressureTreatment OutcomeBed OccupancySupine PositionMuscular AtrophyPlasma VolumeEarly AmbulationMuscle, SkeletalExerciseHypotension, OrthostaticHypovolemiaVascular CapacitanceAmino Acids, EssentialImmobilization