Hindlimb SuspensionSpace FlightHindlimbWeightlessnessMuscular AtrophyWeightlessness SimulationAstronautsMuscle, SkeletalSpace SimulationUnited States National Aeronautics and Space AdministrationMuscle Fibers, Slow-TwitchWeight-BearingMuscle Fibers, Fast-TwitchSuspensionsMyosin Heavy ChainsBed RestOrgan SizeGravity, AlteredAdaptation, PhysiologicalMuscle Fibers, SkeletalAtrophyRats, Sprague-DawleyAerospace MedicineMuscle ProteinsGravitationRats, WistarMuscle ContractionHead-Down TiltBody WeightWeightlessness CountermeasuresTime FactorsHypergravityHypogravityRNA, MessengerEcological Systems, ClosedPleurodelesOrthostatic IntoleranceVestibular Evoked Myogenic PotentialsMusculoskeletal Physiological PhenomenaLocomotionIschemiaCardiovascular Deconditioning