NASA SPRINT exercise program efficacy for vastus lateralis and soleus skeletal muscle health during 70 days of simulated microgravity

The efficacy of the NASA SPRINT exercise countermeasures program for quadriceps (vastus lateralis) and triceps surae (soleus) skeletal muscle health was investigated during 70 days of simulated microgravity. Individuals completed 6° head-down-tilt bedrest (BR, n=9), bedrest with resistance and aerobic exercise (BRE, n=9), or bedrest with resistance and aerobic exercise and low-dose testosterone (BRE+T, n=8). All groups were periodically tested for muscle (n=9 times) and aerobic (n=4 times) power during bedrest. In BR, surprisingly, the typical bedrest-induced decrements in vastus lateralis myofiber size and power were either blunted (MHC I) or eliminated (MHC IIa), along with no change (P>0.05) in %MHC distribution and blunted quadriceps atrophy. In BRE, MHC I (vastus lateralis and soleus) and IIa (vastus lateralis) contractile performance was maintained (P>0.05) or increased (P<0.05). Vastus lateralis hybrid fiber percentage was reduced (P<0.05) and energy metabolism enzymes and capillarization were generally maintained (P>0.05), while not all of these positive responses were observed in the soleus. Exercise offset 100% of quadriceps and ~ ⅔ of soleus whole muscle mass loss. Testosterone (BRE+T) did not provide any benefit over exercise alone for either muscle, and for some myocellular parameters appeared detrimental. In summary, the periodic testing likely provided a partial exercise countermeasure for the quadriceps in the bedrest group, which is a novel finding given the extremely low exercise dose. The SPRINT exercise program appears to be viable for the quadriceps; however, refinement is needed to completely protect triceps surae myocellular and whole muscle health for astronauts on long-duration spaceflights.