Collectively, these studies have shown that muscle strength is reduced by ∼12% within 2 wk of unloading, increasing to ∼30% after 4 mo ( 1, 7). Most of the information on unloaded muscle function in humans has been obtained from ground-based simulation of weightlessness. These data demonstrate that the skeletal muscle of humans becomes weaker and smaller as a result of short- and long-duration space travel.

( 21) reported a 6–8% atrophy in the major leg muscles after an 8-day space shuttle flight. Using magnetic resonance imaging technology, LeBlanc et al. The reduction in muscular strength appears to be accompanied by a decline in whole muscle size. The decline in muscular strength observed in Skylab astronauts and Russian cosmonauts occurred despite in-flight countermeasures (e.g., cycle ergometry, lower body negative pressure). Soviet cosmonauts also experienced a significant decline in ankle extensor strength after short- (7 days) and long-duration (110–237 days) exposure to weightlessness ( 17, 20). Astronauts aboard Skylab (28, 56, and 84 days) averaged a 5–26% decline in whole muscle strength of the knee extensors and flexors ( 29). Nevertheless, marked decrements in muscle strength and size have been reported after spaceflight. Information regarding the functional and morphological changes in human skeletal muscle after spaceflight is limited ( 14), and the results are difficult to interpret because the crew members engaged in physical training during the flights. Because muscle strength is lost during unloading, both during spaceflight and on the ground, these data suggest that the testing sequence employed during the SF and BR may have served as a resistance training countermeasure to attenuate whole muscle strength loss. In summary, no changes in calf muscle strength and morphology were observed after the 17-day SF and BR. Additionally, neither SF nor BR had any effect on fiber composition or fiber size of the calf muscles studied. Maximal isometric calf strength and the force-velocity characteristics were unchanged with SF or BR. Muscle biopsies were obtained before and within 3 h after SF (gastrocnemius and soleus) and BR (soleus) before reloading. Calf muscle strength was examined before SF and BR on days 2, 8, and 12 during SF and BR and on days 2 and 8 of recovery. The subjects' age, height, and weight were 43 ± 2 yr, 183 ± 4 cm, and 86 ± 3 kg for SF and 43 ± 2 yr, 182 ± 3 cm, and 82 ± 4 kg for BR, respectively. The protocols and timelines of these two investigations were identical, therefore allowing for direct comparisons between SF and the BR. The purpose of this investigation was to assess muscle fiber size, composition, and in vivo contractile characteristics of the calf muscle of four male crew members during a 17-day spaceflight (SF Life and Microgravity Sciences Spacelab Shuttle Transport System-78 mission) and eight men during a 17-day bed rest (BR).

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