0.33g mitigates muscle atrophy while 0.67g preserves muscle function and myofiber type composition in mice during spaceflight.

As human space exploration advances, understanding how different gravity levels affect skeletal muscle is critical for long-term health. Among the major organ systems, skeletal muscle is particularly sensitive to gravitational unloading, yet the gravity threshold required to maintain homeostasis remains unclear. Using the Multiple Artificial-gravity Research System aboard the International Space Station, mice were exposed to graded gravity levels, microgravity, 0.33g, 0.67g, and 1g, and their muscles were analyzed postflight. In the gravity-sensitive soleus, the cross-sectional area was preserved at 0.33g, while the slow-to-fast myofiber transition was partially suppressed at 0.33g and fully prevented at 0.67g. Functional measures, including forelimb grip strength and electrical impedance myography, indicated that 0.67g was sufficient to maintain muscle performance. Plasma metabolomics identified 11 metabolites with gravity-dependent changes, suggesting potential biomarkers for monitoring physiological adaptation. Collectively, these results identify 0.67g as a critical threshold for mitigating spaceflight-induced muscle atrophy and myofiber type transitions.