School-based interventions modestly increase physical activity and cardiorespiratory fitness but are least effective for youth who need them most: an individual participant pooled analysis of 20 controlled trials.

Objectives: To determine if subpopulations of students benefit equally from school-based physical activity interventions in terms of cardiorespiratory fitness and physical activity. To examine if physical activity intensity mediates improvements in cardiorespiratory fitness.

Design: Pooled analysis of individual participant data from controlled trials that assessed the impact of school-based physical activity interventions on cardiorespiratory fitness and device-measured physical activity.