AstronautsSpace FlightWeightlessnessCosmic RadiationSpace SuitsSpace SimulationAerospace MedicineWeightlessness SimulationUnited States National Aeronautics and Space AdministrationMusclesSolar ActivityExtravehicular ActivityWeightlessness CountermeasuresSpacecraftMuscle, SkeletalVisible Human ProjectsMuscle ProteinsMuscle, SmoothBed RestLinear Energy TransferOrthostatic IntoleranceMuscle Fibers, SkeletalMuscle, Smooth, VascularBlood Circulation TimeHindlimb SuspensionMuscle DevelopmentMuscle ContractionHypergravityHead-Down TiltHypotension, OrthostaticHeavy Ions