Influence of Maturity Status and Gender on Body Composition and Physical Fitness Adaptations to a Mobile-App-Based Physical Activity Intervention in Adolescents

The use of mobile applications to promote physical activity in adolescents has shown promising results regarding body composition and fitness. However, current evidence is limited by treating adolescents as a homogeneous group, often overlooking whether biological maturity status modulates these effects. This randomized controlled trial analyzed whether biological maturity status and gender modulate the effects of a 10-week step-tracker app intervention in 395 adolescents (13.95 ± 1.20 years). The experimental group (EG) completed walking sessions at least three times per week recorded via the app, while the control group (CG) maintained habitual activities. Somatic maturation was estimated via predicted age at peak height velocity. The 10-week intervention consisted of an app-based step-tracker program (≥3 sessions/week, with goals scaling up to 12,500 steps day). Biological maturity status does not act as a global moderator of the intervention, with its influence being strictly restricted to minor, outcome-specific variations in adiposity metrics among early-maturing males. Males in the EG achieved significant reductions in fat mass (p = 0.002; ηp2 = 0.025) and skinfold thickness (p = 0.005; ηp2 = 0.021) compared to the CG. While physical fitness gains in adolescent males across both groups were predominantly driven by natural pubertal growth, females in the EG demonstrated specific intervention-induced improvements in abdominal muscular endurance (p = 0.001–0.042; ηp2 = 0.011–0.028) and handgrip strength (p = 0.009–0.031; ηp2 = 0.012–0.018) that were absent in controls. In conclusion, biological maturity status and gender show outcome-specific interactive effects on adolescent adiposity and physical fitness, though the associated effect sizes remain consistently small to trivial. Given these results, a standardized mobile step-tracking strategy remains a viable and inclusive tool for educational settings, while the design of gender-tailored digital frameworks should be treated cautiously as a hypothesis for future research.