Muscle Strength, Muscle Morphology, and Oxidative Capacity in Normal Weight versus Overweight and Obese Youth: A Systematic Review with Meta-analysis

Background: The World Health Organization (WHO) defined youth overweight and obesity as pandemic due to a large global overweight and obesity prevalence of 14.8% and 8.1% in 2023. However, the impact of overweight and obesity on youth muscle strength, morphology, and oxidative capacity is unclear despite increased research efforts. Objective: To systematically review the literature and meta-analyze findings on muscle strength, morphology, and oxidative capacity in youth with overweight and/or obesity (OW) versus those with normal-weight (NW). Methods : Following PRISMA guidelines, electronic databases (i.e., PubMed, Web of Science, Scopus, CINAHL, MEDLINE) were searched for cross-sectional studies that compared OW with NW regarding the outcomes muscle strength (e.g., absolute, relative [e.g., to fat-free mass, body mass]), morphology (e.g., muscle cross-sectional area [CSA]), and oxidative capacity (e.g., phase II of volume of oxygen [VO ₂ ] kinetics). The risk of bias (RoB) was assessed using the Joanna Briggs Institute 8-item checklist. A random-effects three-level meta-analysis was used, and within- as well as between-group effect sizes (Hedges’ g [g]) were computed. Study heterogeneity was assessed using I² statistics. The Grading Recommendation Assessment Development and Evaluation (GRADE) approach was used to evaluate the certainty of evidence. Results : Overall, 15 studies were meta-analyzed, including 1,475 participants aged 8.4 to 16.6 years. The mean RoB amounted to 6.1 on a 0 to 8 scale. Muscle absolute strength (n=346; g=0.68; p=0.011; I ² =59.3% [p<0.001]) and CSA (n=1,024; g=1.12; p<0.001; I ² =73% [p<0.001]) was significantly greater in OW versus NW. However, relative muscle strength tended to be larger in NW versus OW (n=320; g=-0.455; p=0.165; I ² =92.7% [p<0.001]). Moreover, muscle oxidative capacity was significantly greater in NW versus OW (n=105; g=-0.58; p=0.019; I ² =37.5% [p=0.18]). GRADE revealed very low (absolute and relative strength and muscle oxidative capacity) to moderate (muscle CSA) certainty of evidence. Conclusions : Absolute muscle strength and CSA seem larger, while relative muscle strength and muscle oxidative capacity seem lower in OW versus NW. The chronic overload imposed by overweight/obesity might lead to greater absolute strength and CSA, although with lower relative muscle strength, suggesting impaired muscle quality related to overweight/obesity. Moreover, impaired oxidative capacity might indicate muscle mitochondrial dysfunction and impaired oxygen supply, with a potential negative impact on exercise tolerance. However, until more high-quality research becomes available, the rather low certainty of evidence precludes a more robust interpretation of findings.