When digital tracking helps or hinders: unpacking the heterogeneous effects of activity trackers on adolescent athletes through the lens of motivation and acceptance

Background Digital activity trackers have become popular in youth training, but the evidence of their effectiveness is mixed. Most of the existing studies rely on the average effect model, ignoring the individual differences in the effect of equipment use and the psychological mechanism behind it. Objective The purpose of this study is to explore the heterogeneous dose–response relationship between the objective use of trackers and the improvement of performance in young football players, and to analyze the mediating role of training load and the moderating role of psychological acceptance and initial physical fitness. Methods The study followed 142 young male football players from 12 youth academies for 12 weeks. We integrated multiple sources of data, including objective usage data recorded by the device (cumulative duration/frequency), performance improvement data (ΔYo-Yo interval recovery test scores), training load data (based on RPE scores), and psychological acceptance data (based on the TAM questionnaire). The quantile regression model was used to analyze the role of each factor under different levels of performance improvement ( τ = 0.10 to 0.90), followed by robustness tests and quantile-specific mediation and moderation analyses. Results There was significant heterogeneity in the association: the association between cumulative duration of use and performance improvement was strongest at the low quantile ( τ = 0.25: B = 2.41, p < 0.001) and not significant at the high quantile ( τ = 0.90: B = −0.21, p = 0.68). Training load was only mediating the association in the middle and lower percentiles ( τ = 0.25 indirect effect = 0.89, 95% CI [0.32, 1.65]). Psychological acceptance positively moderated the association at low performance levels, while initial physical fitness negatively moderated it at low and middle levels, which together explained the observed heterogeneity. The observational design precludes causal inferences. Conclusion The association between tracker use and performance varied significantly among individuals. The potential benefits were most significant for athletes with lower initial fitness levels and higher psychological acceptance, achieved in part through increased training loads. Practice requires personalized application. Due to the observational study design, no causal relationship can be inferred from these findings.