Limited musculoskeletal benefits of artificial gravity combined with cycling during bed rest: Results from the BRACE study

Prolonged exposure to microgravity, simulated via 6° head-down tilt bed rest (HDT), induces musculoskeletal deconditioning and negatively impacts body composition. This study evaluated whether a combination of aerobic exercise with artificial gravity (AG) offers superior protection in comparison to exercise alone. Twenty-four healthy male participants completed 60 days of HDT, randomized into control (C), exercise-only (EX) and exercise with AG (EX-AG) groups. Muscle volume, intramuscular fat, body composition and isokinetic strength were assessed via whole-body MRI and isokinetic dynamometry. All groups experienced thigh fat-free muscle volume loss: C (10.5% ± 2.6%), EX (6.9% ± 2.4%) and EX-AG (4.3% ± 2.4%), with EX-AG showing significantly less atrophy than C (p < 0.001). Compared with C, EX-AG preserved more muscle in both anterior (p < 0.001) and posterior (p < 0.05) compartments, whilst EX preserved more muscle only anteriorly (p < 0.05). The fat ratio increased more in C (8.9% ± 6.0%) compared with EX-AG (-0.8% ± 3.8%; p < 0.05) but not EX (6.5% ± 9.8%). Muscle fat infiltration increased across all groups (C, 7.0% ± 3.7%; EX, 6.2% ± 4.3%; EX-AG, 3.1% ± 4.7%) but was not different between groups (p > 0.05). Maximal isokinetic torque decreased in all groups over all measured angular velocities but was not different between groups (p > 0.05). This is the first study to investigate the combination of AG and exercise as a countermeasure to body composition changes induced by long-term bed rest. We showed that EX-AG provided partial protection against muscle atrophy and fat accumulation but did not outperform exercise alone in preserving muscle quality, strength or overall body composition.