page_title: Reduced muscle mass, strength and performance in space
section_title: International Space Station (ISS)
hierarchical_section_title: Reduced muscle mass, strength and performance in space / Historical overview / U.S. human spaceflight programs / International Space Station (ISS)
caption_attribution_description: English: Figure 6-12. Comparison of postflight percent change in knee extensor and flexor strength from preflight in Shuttle (STS), the three Skylab missions (SL2-4), NASA-Mir (Mir), and ISS.
context_page_description: There is a growing research database which suggests that skeletal muscles, particularly postural muscles of the lower limb, undergo atrophy and structural and metabolic alterations during space flight. However, the relationships between in-flight exercise, muscle changes and performance are not well understood. Efforts should be made to try to understand the current status of in-flight and post-flight exercise performance capacity and what the goals/target areas for protection are with the current in flight exercise program.
context_section_description: The International Space Station's (ISS) first crew (Expedition 1) arrived in October 2000; since then there have been 15 additional Increments.  The data presented here were collected during the first 11 of the ISS Expeditions.
  The complexities and shortcomings of collecting scientific data from a laboratory orbiting more than 300 miles above the Earth and completing 18 orbits per day at a speed of more than 17,000 mph with discontinuous voice and data communications, combined with the constraints and limitations of up mass, crew time, and on-board logistics, cannot be overstated.
  Another problem was exercise hardware that was built and launched but failed to meet science requirements. (The Resistive Exercise Device [RED] science requirement was to provide a load of up to an equivalent of 600 lbs., but the interim RED [iRED] provides only half of that amount. Ground-based studies have shown that it does produce a positive training effect similar to equivalent free weights when used in a high-intensity program, but it will likely not provide sufficient load in a zero-gravity environment to prevent loss of muscle and bone tissue, as determined from parabolic flight studies.) Other problems were failure at one time or another of each piece of onboard exercise hardware with reduced utilization at other times, and other limitations imposed because transmission of forces to the space frame have confounded inflight exercise sessions. In fact, during the first eleven ISS Expeditions, only for 2 short periods during Expeditions 3 and 4 were all three U.S. onboard exercise devices (Cycle Egometer with Vibration Isolation System [CEVIS], Treadmill with Vibration Isolation System [TVIS], and iRED) capable of being used under nominal conditions (Figure 6-10). The almost continuously suboptimal availability of exercise equipment likely has reduced maintenance of crew physical fitness.
  Despite these shortcomings, lean tissue mass data  collected by means of dual-energy x-ray absorptiometry (DEXA) before and after flight compares favorably with data from NASAMir, and the total body and leg losses are in fact less than seen during NASA-Mir or during three separate bed rest studies of similar durations in the range of 20-170 d (Figure 6-11). However, the news is not entirely good since knee extensor and knee flexor strength losses in long-duration crewmembers after flights aboard Mir and ISS were ~23% and ~25%, respectively (Figure 6-12), indicating that strength losses in the quadriceps and hamstring muscle groups were significant and similar for NASA-Mir and early ISS missions, despite apparent slightly increased preservation of muscle mass (lean tissue) in the legs of ISS crewmembers compared to crewmembers on NASA-Mir missions (also Figure 6-11). These near equivalent losses occurred in spite of iRED being present on the ISS. Unfortunately, MRI data collected by Fitts and colleagues to assess skeletal muscle volumes in ISS crewmembers are not yet available to allow comparison with those from NASA-Mir. With respect to endurance, the following comparison (Figure 6-13) shows a trend for improved maintenance of muscle endurance on ISS with respect to NASA-Mir although the loss of endurance on ISS was greater than that documented during short-duration Space Shuttle missions (for ISS, n = 2).
  ISS crewmembers, under the supervision of their crew surgeons, participate in a postflight exercise program implemented by certified trainers who comprise the Astronaut Strength, Conditioning and Rehabilitation (ASCR) group at Johnson Space Center. A portion of this program includes physical fitness testing on an individual basis. The results of these “functional” tests, which consist of six exercises, reveal that crewmembers return with less physical capability than when they launch but that most of the decrements are reversed by postflight day 30 secondary to the ground-based exercises the crewmembers complete in the
  days after their return to Earth (Figures 14 and 15).
  In this section, only the historical highlights of some highly relevant skeletal 
caption: Figure 6-12. Comparison of postflight percent change in knee extensor and flexor strength from preflight in Shuttle (STS), the three Skylab missions (SL2-4), NASA-Mir (Mir), and ISS.
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