MECHANICAL LOADING IS REQUIRED FOR MAINTENANCE OF THE MUSCULOSKELETAL SYSTEM. THUS EXPOSURE TO SPACEFLIGHT OR REDUCED MECHANICAL LOADING ON EARTH INDUCES MARKED BONE LOSS MUSCLE ATROPHY AND DEGRADATION OF SOFT-TISSUE STRUCTURES IN BOTH THE KNEE (E.G. CARTILAGE MENISCI AND LIGAMENTS) AND HIP (E.G. CARTILAGE) JOINTS. THIS IS A MAJOR CONCERN FOR ASTRONAUTS DURING AND AFTER LONG-DURATION SPACEFLIGHT AS THEY MAY BE AT INCREASED RISK FOR REDUCED PERFORMANCE BONE FRACTURES AND BOTH EARLY-ONSET OSTEOPOROSIS AND ARTHRITIS. ARTIFICIAL GRAVITY VIA CENTRIPETAL ACCELERATION IS A POSSIBLE APPROACH TO MITIGATE THESE DELETERIOUS CHANGES. YET THE ABILITY OF PARTIAL GRAVITY INDUCED BY CENTRIPETAL ACCELERATION TO INHIBIT ADVERSE MUSCULOSKELETAL CHANGES IN SPACEFLIGHT REMAINS UNKNOWN. GIVEN THE CONSTRAINTS OF STUDYING CENTRIFUGATION AS A COUNTERMEASURE ON EARTH SPACEFLIGHT-BASED STUDIES ARE NEEDED. WE PROPOSE TO DETERMINE THE EFFECTS OF VARYING PARTIAL GRAVITY LEVELS ON BONE MUSCLE AND SOFT TISSUES OF THE HIP AND KNEE JOINTS IN ADULT MICE FLOWN ABOARD THE INTERNATIONAL SPACE STATION (ISS) IN THE JAXA MOUSE HABITAT UNIT. WE WILL EXAMINE BONE STRUCTURE POST-FLIGHT USING HIGH-RESOLUTION MICROCOMPUTED TOMOGRAPHY (MICROCT); BONE CELLULARITY USING QUANTITATIVE HISTOMORPHOMETRY; BONE FUNCTION VIA BIOMECHANICAL TESTING AND NANO-INDENTATION; AND BONE COMPOSITION VIA RAMAN SPECTROSCOPY AND QUANTITATIVE BACKSCATTERED ELECTRON IMAGING. WE WILL EXAMINE NEUROMUSCULAR FUNCTION VIA PRE- AND POST-FLIGHT ROTOROD TESTING GAIT ANALYSIS AND GRIP STRENGTH MEASUREMENTS. POST-FLIGHT MUSCLE ANALYSES WILL INCLUDE HISTOLOGY AND ELECTRICAL IMPEDANCE MYOGRAPHY. POST-FLIGHT ANALYSES OF JOINT SOFT-TISSUES WILL INCLUDE STRUCTURAL MEASUREMENTS OF CARTILAGE MENISCI AND LIGAMENTS USING BOTH CONTRAST-ENHANCED HIGH-RESOLUTION MICROCT AND HISTOLOGY; MOLECULAR COMPOSITION OF CARTILAGE AND MENISCI USING PROTEOMICS AND RAMAN SPECTROSCOPY; AND BIOMECHANICAL PROPERTIES OF CARTILAGE USING NANO-INDENTATION. CELLULAR AND MOLECULAR RESPONSES FOR BONE MUSCLE AND JOINT SOFT TISSUES WILL ALSO BE EVALUATED VIA WHOLE TRANSCRIPTOME ANALYSES (E.G. RNASEQ). RESULTS FROM THESE INTEGRATED COMPREHENSIVE ANALYSES WILL PROVIDE INFORMATION REGARDING WHETHER PARTIAL GRAVITY EITHER INDUCED BY CENTRIPETAL ACCELERATION OR VIA MOON OR MARS ENVIRONMENTS WILL PROTECT FROM MUSCULOSKELETAL DETERIORATION DURING SPACEFLIGHT OR WHETHER ADDITIONAL COUNTERMEASURES WILL BE NECESSARY.