Physical activity, mental health, and healthy development in adolescents: mechanisms and implications for intervention.

Adolescence combines heightened neurobehavioral plasticity with a steep rise in the incidence of depressive and anxiety disorders. Habitual physical activity, by contrast, begins to fall much earlier, from about age seven, so adolescents enter the years of peak diagnostic risk already substantially less active than they were as children. That offset, rather than a coincidence of two curves, is the premise of this review. Meta-analytic syntheses report moderate improvements in depressive (SMD = -0.45, 95% CI -0.59 to -0.31) and anxiety symptoms (SMD = -0.39, 95% CI -0.61 to -0.17) after exercise interventions in samples aged 5 to 18 years, yet several large pragmatic trials have detected no benefit. This mini review synthesizes recent evidence on these effects, the mechanisms proposed to explain them, and why the evidence base remains internally inconsistent. We argue that the contradiction is largely resolvable: efficacy estimates come from a heavily overlapping pool of trials and are inflated by unblinded designs, self-reported outcomes, and passive comparators, whereas universal school-based trials recruit predominantly asymptomatic students and deliver far less activity than prescribed. Mechanisms span three nested levels, neurobiological, psychosocial and behavioral, but most mediation evidence is concurrent rather than temporally ordered. Read through self-determination theory and acceptance-based accounts of distress tolerance, tolerable difficulty appears to be part of how participation confers benefit, with implications for injury, dropout and elite youth sport. Priorities include adequately powered trials with active controls and process evaluation, compositional analysis of movement behaviors, and temporally ordered mechanistic designs.