The effect of bed rest, unilateral limb immobilization and head-down tilt on muscle protein synthesis: A systematic review and meta-analysis.

Muscle disuse leads to muscle atrophy and a decrease in muscle function that is primarily driven by reduced muscle protein synthesis (MPS). The aim of this systematic review and meta-analysis was to examine the effect of different models of muscle disuse on rates of MPS. A literature search was conducted in PubMed, Web of Science, Scopus and Cochrane Library. Eligible randomized and non-randomized controlled trials compared the effect of bed rest (BR), unilateral lower limb immobilization (ULLI), and 6° head-down tilt (HDT) on pre-post muscle disuse changes in MPS in adults. Using the random and fixed effects inverse-variance model, we calculated the mean difference (MD) in the effect of BR on mixed MPS and ULLI on myofibrillar MPS (MyoPS) (both expressed as fractional synthetic rate,%/h). The effect of HDT on MPS was examined through a narrative synthesis of data. A total of 16 studies were included in the systematic review and 13 in the meta-analysis. A significant reduction in mixed MPS was observed after BR (k = 4; MD: -0.017%/h, 95% CI: -0.023 to -0.011, I2 = 24%, P < 0.01) and a significant reduction in MyoPS was observed after ULLI (k = 9; MD: -0.015%/h, 95% CI: -0.021 to -0.008, I2 = 94%, P < 0.01). HDT led to reductions in both mixed MPS and MyoPS. A comparable reduction in mixed MPS and MyoPS was observed between different models of muscle disuse.