0.33 <i>g</i> mitigates muscle atrophy while 0.67 <i>g</i> 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, 0.33<i>g</i>, 0.67<i>g</i>, and 1<i>g</i>, and their muscles were analyzed postflight. In the gravity-sensitive soleus, the cross-sectional area was preserved at 0.33<i>g</i>, while the slow-to-fast myofiber transition was partially suppressed at 0.33<i>g</i> and fully prevented at 0.67<i>g</i>. Functional measures, including forelimb grip strength and electrical impedance myography, indicated that 0.67<i>g</i> 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 0.67<i>g</i> as a critical threshold for mitigating spaceflight-induced muscle atrophy and myofiber type transitions.