Adiposity, Fat-Free Mass Index, and Muscular Strength in Children: Independent Effects on Functional Performance in a Tertiary Pediatric Endocrinology Cohort

Background and Objectives: Childhood obesity is associated with alterations in body composition that may impair muscular strength and functional capacity. While higher body mass is often accompanied by greater absolute strength, the independent effect of adiposity on muscle strength after accounting for lean mass remains insufficiently understood. This study aimed to evaluate the associations between adiposity and muscle strength in children and adolescents, while accounting for growth and maturation, and to examine differences according to weight status. Materials and Methods: This retrospective cross-sectional study included 84 children and adolescents aged 5–18 years. Anthropometric measurements were used to calculate body mass index (BMI), waist-to-hip ratio, and waist-to-height ratio, with weight status classified according to CDC BMI-for-age percentiles. Body composition was assessed using bioelectrical impedance analysis (Tanita). Pubertal stage was evaluated using Tanner classification. Muscle strength was assessed using dominant handgrip strength. Associations between adiposity-related parameters and muscle strength were analyzed using correlation and multivariable linear regression models adjusted for age, sex, pubertal stage, physical activity, and body composition. Results: Body mass index was positively correlated with absolute handgrip strength (r = 0.561, p < 0.001). Body fat percentage was negatively associated with relative handgrip strength (r = −0.381, p < 0.001). In multivariable regression analyses, body fat percentage remained an independent negative predictor of handgrip strength (β = −0.203, p = 0.0046), whereas fat-free mass and fat-free mass index were positive predictors in respective models (p < 0.001). Conclusions: Increased adiposity is associated with reduced muscle strength in children and adolescents when strength is evaluated relative to body size or adjusted for lean mass. These findings support the concept of impaired muscle performance in pediatric populations with excess adiposity and highlight the importance of integrating body composition and functional assessments in clinical evaluation.