Early structural and functional signature of 3-day human skeletal muscle disuse using the dry immersion model

Microgravity and hypoactivity are associated with skeletal muscle deconditioning. The decrease of muscle mass follows an exponential decay, with major changes in the first days. The purpose of the study was to dissect out the effects of a short-term 3-day dry immersion (DI) on human quadriceps muscle function and structure. The DI model, by suppressing all support zones, accurately reproduces the effects of microgravity. Twelve healthy volunteers (325years) completed 3days of DI. Muscle function was investigated through maximal voluntary contraction (MVC) tests and muscle viscoelasticity. Structural experiments were performed using MRI analysis and invasive experiments on muscle fibres. Our results indicated a significant 9.1% decrease of the normalized MVC constant (P=0.048). Contraction and relaxation modelization kinetics reported modifications related to torque generation (k(ACT)=-29%; P=0.014) and to the relaxation phase (k(REL)=+34%; P=0.040) after 3days of DI. Muscle viscoelasticity was also altered. From day one, rectus femoris stiffness and tone decreased by, respectively, 7.3% (P=0.002) and 10.2% (P=0.002), and rectus femoris elasticity decreased by 31.5% (P=0.004) after 3days of DI. At the cellular level, 3days of DI translated into a significant atrophy of type I muscle fibres (-10.612.1%, P=0.027) and an increased proportion of hybrid, type I/IIX fibre co-expression. Finally, we report an increase (6-fold; P=0.002) in NCAM+ muscle fibres, showing an early denervation process. This study is the first to report experiments performed in Europe investigating human short-term DI-induced muscle adaptations, and contributes to deciphering the early changes and biomarkers of skeletal muscle deconditioning.