Skeletal muscle and cardiorespiratory responses to simulated microgravity

As a part of a project designed to mimic a Space Shuttle flight (LMS), eight healthy males completed 17 d of -6° head down tilt bedrest to assess the in vivo torque velocity relationship of the calf muscle group and the cardiorespiratory responses to supine cycling exercise. The subjects age, height, and weight were 42.7 ±8.1 y, 182.3 ±6.5 cm, and 82.2 ±12.1 kg, respectively. Testing was completed prior to bedrest (CON), on bedrest days 2 & 3 (BR 1), 8 & 9 (BR2), and 13 & 14 (BR3), as well as recovery days 3 & 4 (R1), and 7 & 8 (R2). Maximal plantar flexion force production at seven angular velocities (0, 30, 60, 120, 180, 240, 300'- s-1) was unchanged (P>0.05) during bedrest and recovery. Muscle biopsy specimens obtained from the soleus before and on day 17 of bedrest showed no change (P>0.05) in muscle fiber composition, muscle fiber area, capillary to fiber ratio, or capillary density. Citrate synthase activity decreased 21% (P<0.05) from 122.1 ±7.8 to '97.1 ±5.1 µmol -min-1 • g dry wt-1, while phosphorylase activity was unchanged (P>0.05). VO2max (L • min-1) was decreased (P<0.05) from CON (3.24 ±0.20) during BR1 (2.99 ±0.17; -7.3%), BR2 (3.00 ±0.17; -7.1%), BR3 (2.92 ±0.20; -9.0%), and R1 (3.02 ±0.20; -6.6%), but was not different (P>0.05) than CON by R2 (3.13 ±0.19; -3.3%). Maximal heart rate and ventilation did not change (P>0.05) from CON during bedrest or recovery. Initial changes in VO2max (BR1) were significantly correlated with fluid balance during bedrest day 1 (r=0.91, P<0.05). These data suggest that the testing protocols in this investigation may be sufficient to attenuate functionally significant changes in muscle morphology and strength during 17 d of simulated microgravity. The results also suggest that the time course for changes in VO2max are not linear and are related to the initial changes in body fluid volumes.