NEUROMUSCULAR AND MYOTENDINOUS ALTERATIONS IN-FLIGHT IN PLANTAR FLEXOR MUSCLES

BACKGROUND Despite extensive exercise countermeasures, astronauts still experience sizable deficits in calf muscle mass and strength. Recent muscle biopsy data suggest deficits are associated with metabolic deviations [1]. Many studies have tested astronaut strength before and after spaceflight. The one study that tested in-flight reported a significant reduction in isometric and isokinetic knee extensor strength that was pronounced from the very beginning of the space sojourn [2]. This is in stark contrast to bed rest studies, where a gradual and continuous loss of muscle mass and strength has been repeatedly reported. METHODS The Sarcolab3 study attempted to describe the time course of changes in isometric and isokinetic muscle strength and in muscle mass. Crew members were tested on board the ISS after 8.9 (SD 3.0), 49.1 (10.1) and 131.5 (SD 27.7) days in-flight, as well as pre-and post-flight, using the Muscle Atrophy Research and Exercise System (MARES). Specifically, maximal voluntary torques in plantar flexion and knee extension were measured at various angles in order to establish the torque-angle relationship. Simultaneously, activation capacity was assessed by supra-maximal myo-electrical stimulation superimposed to a maximum voluntary contraction. Calf muscle volume was assessed by magnetic resonance imaging pre-and post-flight. RESULTS Results demonstrate early declines in muscle strength at flight days 9 and 49, and a tendency for the decaying time course to level off at flight day 131. Activation capacity of plantar flexor, as elicited by myo-electrical stimulation, was maintained during in-flight and post-flight testing. At 4 days post-flight, soleus muscle volume had declined by 15-20% as compared to pre-flight. CONCLUSIONS Results suggest that existing exercise countermeasures are still not fully effective for preventing weakness and wasting of the calf musculature. The observed time course of in-flight muscle loss seems more compatible with results from bed rest. Potentially, the rate of muscle wasting may decline during the second half of 6-month missions with the current countermeasure exercise regimens. REFERENCES 1. Rittweger, J., et al., Sarcolab pilot study into skeletal muscle's adaptation to long-term spaceflight. NPJ Microgravity, 2018. 4: p. 18. 2. Tschan, H., et al., Resistance training - a strategy proposed to coutneract muscular deconditioning in microgravity, in 10 Years Space Biomedical Research and Development in Austria, H. Hinghofer-Szalkay, Editor. 2001, Facultas: Wien. p. 95-108