Exercise Countermeasures for Long Duration Space Missions: Considerations for Muscle Specific Adaptations to Cycle Exercise: 2893

Studies of real and simulated microgravity exposure show the muscles of the lower limbs atrophy to the greatest extent, with the calf muscles being the most affected and the most difficult to target with exercise countermeasures. PURPOSE: To determine the involvement of the thigh and calf muscles during two cycle exercise protocols (low and high intensity) central to the exercise countermeasures program on the International Space Station (ISS). METHODS: Intramuscular glycogen and triglyceride levels were quantified in the quadriceps (vastus lateralis) and triceps surae (soleus) muscles before and after a low intensity (current ISS prescription: 45 minutes at 55% VO2max, 131±12W) and a high intensity (proposed ISS prescription: 8×30 second intervals at 150% VO2max, 459±34W) cycle exercise bout in nine men and women. RESULTS: During low intensity cycling, glycogen was reduced in the vastus lateralis (114±27 mmol/kg dry wt, p<0.05) and remained unchanged in the soleus (p>0.05). High intensity cycling reduced glycogen in both muscles (p<0.05), but the reduction in the soleus (59±11 mmol/kg dry wt) was ∼60% less (p<0.05) than the vastus lateralis (151±25 mmol/kg dry wt). Intramuscular triglycerides were unchanged in both muscles at both intensities (p>0.05). CONCLUSION: Low intensity cycle exercise does not likely contribute to preserving calf muscle health during long duration exposure to microgravity. These findings, coupled with other ground-based studies, provide strong support for high intensity cycling being a more appropriate component of the ISS prescription for upper and lower leg skeletal muscle health and cardiorespiratory fitness, although additional exercise paradigms that target the calf are warranted. These muscle specific findings should be considered when designing exercise strategies for combating conditions of sarcopenia and muscle wasting on Earth. Supported by NIH AG020532 and NASA NNJ06HF59G.