This study was designed to examine the effects of weekday and weekend moderate-to-vigorous physical activity (MVPA) at age 14 on internalizing and externalizing problems at age 17 in 1,238 adolescents taking part in the UK Millenium Cohort Study who were overweight or obese. Accelerometer data were collected on one weekday and one weekend day when participants were 14 years old. Outcomes at age 17 were parent-reported Strengths and Difficulties Questionnaire (SDQ) internalizing (emotional + peer problems) and externalizing (conduct + hyperactivity/inattention) composites. Covariate-adjusted average treatment effects (ATEs; per 1 min/day MVPA) were small and not statistically significant for internalizing or externalizing problems. Conditional average treatment effects (CATEs, heterogeneous effects) were examined and height was found to be the strongest moderator (β = 0.0002; p < 0.001; R² = 0.519) of weekday MVPA effects internalising (emotional and peer- problems) with and more negative CATEs among shorter adolescents (Q1 −0.004994 vs Q4 −0.001903; ANOVA F = 106.741, p < 0.001). In summary, the results for this cohort of adolescents with overweight/obesity, showed estimated average effects of weekday and weekend MVPA at the age of 14 on parent-reported internalizing and externalizing problems at age 17 were close to zero under a selection-on-observables framework. The results suggest potential benefits may be subgroup-specific (e.g. height) and context-dependent.
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