International Space Station .28ISS.29 Reduced muscle mass, strength and performance in space

figure 6-10. exercise equipment failures , other constraints have limited access of iss crewmembers full complement of aerobic , resistance exercise protocols. full capability 3 devices present 2 short windows during expeditions 3 , 4 (tall white rectangles).

another problem exercise hardware built , launched failed meet science requirements. (the resistive exercise device [red] science requirement provide load of equivalent of 600 lbs., interim red [ired] provides half of amount. ground-based studies have shown produce positive training effect similar equivalent free weights when used in high-intensity program, not provide sufficient load in zero-gravity environment prevent loss of muscle , bone tissue, determined parabolic flight studies.) other problems failure @ 1 time or of each piece of onboard exercise hardware reduced utilization @ other times, , other limitations imposed because transmission of forces space frame have confounded inflight exercise sessions. in fact, during first eleven iss expeditions, 2 short periods during expeditions 3 , 4 3 u.s. onboard exercise devices (cycle egometer vibration isolation system [cevis], treadmill vibration isolation system [tvis], , ired) capable of being used under nominal conditions (figure 6-10). continuously suboptimal availability of exercise equipment has reduced maintenance of crew physical fitness.

figure 6-11. lean tissue mass losses in percent change preflight nasa-mir, iss, , 3 bed rest studies 120-170 days in duration.

despite these shortcomings, lean tissue mass data collected means of dual-energy x-ray absorptiometry (dexa) before , after flight compares favorably data nasamir, , total body , leg losses in fact less seen during nasa-mir or during 3 separate bed rest studies of similar durations in range of 20-170 d (figure 6-11). however, news not entirely since knee extensor , knee flexor strength losses in long-duration crewmembers after flights aboard mir , iss ~23% , ~25%, respectively (figure 6-12), indicating strength losses in quadriceps , hamstring muscle groups significant , similar nasa-mir , iss missions, despite apparent increased preservation of muscle mass (lean tissue) in legs of iss crewmembers compared crewmembers on nasa-mir missions (also figure 6-11). these near equivalent losses occurred in spite of ired being present on iss. unfortunately, mri data collected fitts , colleagues assess skeletal muscle volumes in iss crewmembers not yet available allow comparison nasa-mir. respect endurance, following comparison (figure 6-13) shows trend improved maintenance of muscle endurance on iss respect nasa-mir although loss of endurance on iss greater documented during short-duration space shuttle missions (for iss, n = 2).

figure 6-12. comparison of postflight percent change in knee extensor , flexor strength preflight in shuttle (sts), 3 skylab missions (sl2-4), nasa-mir (mir), , iss.

iss crewmembers, under supervision of crew surgeons, participate in postflight exercise program implemented certified trainers comprise astronaut strength, conditioning , rehabilitation (ascr) group @ johnson space center. portion of program includes physical fitness testing on individual basis. results of these “functional” tests, consist of 6 exercises, reveal crewmembers return less physical capability when launch of decrements reversed postflight day 30 secondary ground-based exercises crewmembers complete in days after return earth (figures 14 , 15).

figure 6-13. postflight (r+0) percent change preflight measures in muscle endurance @ knee expressed total work space shuttle (sts), nasa-mir (mir) , iss (for iss, n = 2).

in section, historical highlights of highly relevant skeletal muscle investigations have been included , discussed. complete treatment of data cover several volumes. however, brief historical overview possible see how initial indications of losses in skeletal muscle function led attempts provide exercise countermeasures. such countermeasures utilized during spaceflight, crewmembers tested upon return, , exercise regimens , equipment modified use in future missions. in subsequent sections, human spaceflight , ground-based analog studies , experimental animal studies reviewed contribute evidence base on alterations in skeletal muscle form , function occur muscle unloading associated microgravity environment. knowledge base on future operational countermeasures , investigations fundamental changes in muscle physiology based.

figure 6-14. results of functional fitness testing in crewmembers iss expeditions 1-11. percent change before flight @ postflight days 5/7 , 30.



figure 6-15. results of functional fitness testing in crewmembers iss expeditions 1-11. percent change before flight @ postflight days 5/7 , 30.