MRI assessment of leg muscle size and composition after a 7-day head-down bed rest

Objective: To investigate the effects of 7-day −6° head-down bed rest (HDBR) on lower limb muscle volume, fat, and muscle strength in healthy males. Materials and Methods: In this prospective study, thigh and calf proton density fat fraction (PDFF) magnetic resonance imaging (MRI) was performed on 34 (thigh-measured) and 32 (calf-measured) healthy male participants. These participants were subjected to the HDBR trial between July 2024 and September 2024 at five time points: baseline (PRE), bed rest day 3 (BR3), day 7 (BR7), and recovery days 3 (R3) and 5 (R5). Muscle volume, subcutaneous fat volume, muscle PDFF, and subcutaneous fat PDFF were quantified. Blood and urine samples were collected to measure biomarkers. Analysis of dynamic changes in indicators of thigh and calf parameters and their correlation with biomarkers. Results: A total of 34 healthy male participants underwent HDBR. 34 (mean age, 28.03 ± 6.74 [SD]) had thigh measurements, and 32 (mean age, 27.47 ± 5.79 [SD]) had calf measurements. Bilateral thigh muscle cross-sectional area (MCSA) and bilateral medial head of gastrocnemius muscle cross-sectional area (MHMCSA) decreased consistently and significantly during HDBR (p<0.001). Thigh subcutaneous adipose tissue (SAT) and calf subcutaneous adipose tissue (CSAT) areas initially decreased, then gradually increased during HDBR, with inflection points at BR3. Left thigh subcutaneous adipose tissue fat fractions (SATL-FF) and left calf subcutaneous adipose tissue fat fractions (CSATL-FF) increased consistently and significantly during HDBR (p<0.001). The right medial head of the gastrocnemius muscle intramuscular fat fraction (MHIMFR-FF) increased significantly during HDBR (p=0.035). Significant correlations (p<0.05) were observed between urinary cortisol, cortisol, red blood cells, and calcium with muscle/fat parameters. Conclusion: Short-term HDBR induces lower limb muscle atrophy and subcutaneous fat redistribution, which are dynamically correlated with stress, metabolic, and bone metabolism-related biomarkers.