Longitudinal alterations in lower extremity quantitative MRI metrics under simulated microgravity and their associations with systemic biomarkers

This study employed a before-and-after design and utilized proton density fat fraction (PDFF) magnetic resonance imaging (MRI) to investigate the effects of 7 days of -6° head-down tilt bed rest (HDBR) on the lower limb muscles of healthy men. We performed multiple MRI scans of the participants’ thighs and calves during the HDBR period and collected blood and urine samples at corresponding time points to analyse biomarkers. We observed a sustained and significant reduction in thigh muscle cross-sectional area (TMCSA) and medial head of the gastrocnemius muscle cross-sectional area (MHMCSA) during HDBR ( p < 0.001). The thigh subcutaneous adipose tissue area (TSAT) and the calf subcutaneous adipose tissue area (CSAT) showed a downward trend at the onset of HDBR, followed by a gradual recovery. During HDBR, both the thigh and calf muscle contractile mass index (CMI) exhibited a significant and sustained downward trend. Phosphorus (P), direct renin (DR), and red blood cells (RBC) showed significant correlations with muscle/fat parameters ( p < 0.05). Our findings indicate that short-term HDBR leads to lower limb muscle atrophy and subcutaneous fat redistribution, and that these changes are dynamically associated with stress responses, metabolic indicators, and biomarkers related to bone metabolism.