Longitudinal Links Between Muscular Fitness, Physical Activity, Body Composition, and Sports Club Participation Across Childhood and Adulthood in Germany

Purpose: Muscular fitness represents a central component of physical fitness, associated with lower adiposity, better cardiometabolic and psychosocial health, and reduced mortality risk. It tracks from childhood into adulthood and is shaped by behavioral and physiological factors. This study investigated how physical activity, body composition, and sports club participation contribute to muscular fitness development from childhood to adulthood in a large national sample. Methods: Longitudinal data from participants aged 4–31 years from the MoMo Study (2003–2017) were analyzed. Muscular fitness, physical activity, body fat percentage, waist-to-height ratio, and sports club participation were assessed using validated tests, questionnaires, and bioelectrical impedance analysis. Directional associations were examined using cross-lagged panel models. Results: The analysis showed small to large autoregressive effects across all variables (\((\beta)\) = 0.096–0.957). Higher physical activity predicted higher subsequent Muscular fitness and performance (\((\beta)\) = 0.026–0.210), while higher body fat percentage predicted lower later Muscular fitness and performance (\((\beta)\) = –0.051 to –0.297). Sports club participation was also associated with higher subsequent Muscular fitness and performance (\((\beta)\) = 0.027–0.185). Associations from muscular fitness to later physical activity, body composition, and sports club participation were weaker and less consistent. Conclusion: Muscular fitness and body composition showed high stability from childhood to early adulthood. Cross-lagged analyses indicate body fat percentage exerts stronger influences on Muscular fitness development than activity indicators. The results demonstrate that activity indicators and body fat percentage drive long-term fitness development, underscoring the need for early interventions to prevent persistent low muscular fitness and unfavorable body composition.