Lumbar muscle atrophy and increased relative intramuscular lipid concentration are not mitigated by daily artificial gravity after 60-day headdown tilt bed rest

Exposure to axial unloading induces adaptations in paraspinal muscles, as shown after spaceflights. This study investigated whether daily exposure to artificial gravity (AG) mitigated lumbar spine flattening and muscle atrophy associated with 60-day head-down tilt (HDT) bed rest (Earth-based space analog). Twenty-four healthy individuals participated in the study: 8 received 30-min continuous AG; 8 received 6 × 5-min AG interspersed with rest periods; and 8 received no AG exposure (control group). Magnetic resonance imaging (MRI) of the lumbopelvic region was conducted at baseline (BDC) and at day 59 of HDT (HDT59). Longitudinal relaxation time (T1)-weighted images were used to assess morphology of the lumbar spine (spinal length, intervertebral disk angles, disk area) and volumes of the lumbar multifidus (LM), lumbar erector spinae (LES), quadratus lumborum (QL), and psoas major (PM) muscles from L1/L2 to L5/S1 vertebral levels. A chemical shift-based two-point lipid/water Dixon sequence was used to evaluate muscle composition. Results showed that spinal length and disk area increased (P < 0.05); intervertebral disk angles (P < 0.05) and muscle volumes of LM, LES, and QL reduced (P < 0.01); and lipid-to-water ratio for the LM and LES muscles increased (P < 0.01) after HDT59 in all groups. Neither of the AG protocols mitigated the lumbar spinae deconditioning induced by HDT bed rest. The increase in lipid-to-water ratio in LM and LES muscles indicates an increased relative intramuscular lipid concentration. Altered muscle composition in atrophied muscles may impair lumbar spine function after body unloading, which could increase injury risk to vulnerable soft tissues. This relationship needs further investigation.