Medial gastrocnemius activation during graded axial hypergravity induced by short-arm human centrifugation

BACKGROUND: Gravitational loading is a key determinant of antigravity muscle activation, yet neuromuscular responses to graded + Gz loading in humans remain incompletely characterized. Reduced mechanical loading, whether due to microgravity or physical inactivity, leads to musculoskeletal deconditioning across both spaceflight and clinical contexts. METHODS: Twenty-five adults with stable neuromuscular or neurodegenerative disorders and twenty-one age-matched healthy controls completed a short-arm human centrifugation protocol under a no-rotation baseline condition (ω = 0) and at 1.5, 1.7, and 2.0 g measured at the feet. Surface electromyography (sEMG) of the medial gastrocnemius was recorded for 60 s at each condition and analyzed as log-transformed root mean square amplitude using linear mixed-effects models. RESULTS: Medial gastrocnemius EMG increased significantly at all hypergravity levels compared with the no-rotation baseline (main effect of condition, p ≤ 0.001). No significant main effect of group was observed, although a small group × condition interaction indicated modest differences in the magnitude of response. Within-session reliability was excellent, whereas between-day reliability was moderate. Cardiovascular parameters, assessed in a subset of participants, remained within physiological ranges across conditions. CONCLUSIONS: + Gz loading induces reproducible increases in antigravity muscle activation in humans, in both groups of patients and healthy controls. These findings demonstrate that short-arm centrifugation provides a controllable mechanical stimulus for investigating load-dependent neuromuscular activation and human integrative responses to altered gravitational loading.