Imbalance between muscle strength development and weight gain in children and young adults in China: serial cross-sectional evidence from 1.33 million students from five successive national surveys between 2000 and 2019.

Background: Over the past decades, Chinese school-aged children and adolescents have experienced a rapid increase in overweight and obesity, but corresponding changes in muscle strength remain unclear. We aimed to elucidate trends in muscle strength development at both national and subnational level, and to investigate whether it has increased proportionally with weight gain. Methods: Muscle strength (handgrip strength [kg] for upper body and standing broad jump [cm] for lower body) and weight measurements for 1.33 million children and adolescents aged 7 to 22 were obtained from five waves of the Chinese National Surveys on Students Constitution and Health (2000, 2005, 2010, 2014, and 2019). Trends in muscle strength indicators were investigated using t-test, with post-hoc pairwise comparisons performed using Bonferroni correction. Associations between muscle indicators with weight change were assessed with the Pearson correlation analysis. Findings: Despite some increase during the first waves of survey, both handgrip strength and standing broad jump declined significantly between 2010 and 2019. The median handgrip strength decreased from 43.9 kg (95% CI 43.7 to 44.1) in 2010 to 42.5 kg (42.3 to 42.8) in 2019 among young males, and from 26.6 kg (26.5 to 26.7) in 2010 and to 26.0 kg (25.8 to 26.1) in 2019 among young females. From 2010 to 2019, median standing broad jump decreased from 234.3 cm (233.8-234.8) to 219.4 cm (218.8-220.1) among males, and from 172.8 cm (172.3-173.3) to 162.5 cm (162.0-163.1) among females. Over the last decade, both measures of muscle strength showed significant declines within each age- and sex-specific quantiles, with the most pronounced decreases observed at the lower quantiles and older ages. The most notable declines were observed in North China, Northeast China, and Middle-West China. As age and weight increased, the initially positive association between body weight and handgrip strength shifted to a progressive weakening, while the association between body weight and standing broad jump changed from non-significant to negative, at -0.51 (95% CI: -0.62 to -0.38) for males and -0.39 (95% CI: -0.52 to -0.24) for females. Interpretation: This nationally representative study highlights a significant decline in muscle strength among Chinese students over the last decade. The imbalance between increasing body weight and declining muscle strength underscores the urgent need for targeted public health interventions to decrease obesity in Chinese students, to maximize their life-long health expectancy. Funding: This study was supported by the National Natural Science Foundation of China (82073573 to ZZ; 82204067 to XW; 82273654 to YS).