0.67 g Threshold: A Gravity Benchmark for Sustaining Muscles in Space

A study on mice aboard the ISS exposed them to 0.33 g, 0.67 g, and 1 g for up to 28 days to assess muscle atrophy. It found 0.67 g is the threshold below which muscles deteriorate, while 0.33 g preserved muscle but changed fiber composition; while humans may share a similar threshold (roughly 0.5–0.75 g) based on parabolic-flight data, Moon and Mars gravity (0.17 g and 0.38 g) would likely require artificial gravity or stronger countermeasures for long missions, with further research needed to refine the human threshold and inform NASA's Artemis-era plans.