Quantifying the Toll of Disuse: A Meta-Analysis of Skeletal Muscle Mass and Strength Loss Following Upper Limb Immobilization.

Enhancing our understanding of the specific characteristics that disuse-induced models should possess, based on the immobilized joint, could significantly enhance the effectiveness of research in this field. Objective: Our objective was to quantify the decrease in skeletal muscle mass and strength in humans subjected to upper limb disuse-induced models. Methods: PubMed, Scopus, Web of Science, Embase, LILACS, SPORTDiscus, CINAHL, and Epistemonikos databases were searched from inception to November 2025. Randomized controlled trials, cross-over clinical trials, or quasi-experimental studies performed in healthy adults ≥18 years old, subjected to an induced-disuse model to investigate the effects on skeletal muscle mass or strength were included. Results: Forty-five studies were included. Significant differences in skeletal muscle mass, equivalent to a small effect size (SMD = -0.453; 95% CI = -0.698 to -0.208; p < 0.001) and a total loss of 3.44% (estimated average daily decline = 0.16%) were observed after 21 days of immobilization. Skeletal muscle mass loss was heterogeneous between the arm (5.03%), forearm (1.56%), and hand (4.67%) muscles. Significant differences in strength, equivalent to a large effect size (SMD = -1.36; 95% CI = -1.69 to -1.02; p < 0.001) and a total loss of 18.06% (estimated average daily decline = 1.55%), were observed after 18 days of immobilization. Strength decreases were heterogeneous between the arm (16.67%), forearm (21.42%), and hand (10.46%) muscles. Conclusion: Based on evidence of very low certainty, upper limb disuse-induced models appear to induce a nonlinear loss of skeletal muscle mass and a suggested substantially more severe and rapid loss of strength in healthy young adults, with effects varying heterogeneously across different muscle groups. Despite data limitations, these estimates provide a basis for designing experimental countermeasures, though caution is warranted due to the heterogeneity of the findings.