Background The positive effect of physical activity (PA) on mental health in adults is well-established, but less is known about this relationship in young children. The aim is to explore the nature of the association between objectively-measured PA and mental health in UK Millennium Cohort Study children, and identify groups in which this relationship might differ.

Methods Accelerometer data were collected at age 7 years. Mental health at ages 7 and 11 was measured using the four difficulties subscales (peer, emotional, conduct, hyperactivity) of the parent-rated Strengths and Difficulties Questionnaire (SDQ). Associations between the mean daily number of minutes adjusted for wear time at different activity intensities (sedentary, light, moderate, vigorous) and SDQ outcomes at age 11 (score range 0–10) were estimated using multiple linear regression models, adjusting for SDQ at age 7, individual and family characteristics and stratified by gender (N = 6,153). All models were stratified by gender, while models measuring effects of sedentary time were additionally stratified by whether children were active or not (i.e. meeting the UK guidelines of at least 60 mins of moderate-to-vigorous PA/day).

Results In fully adjusted models, increased PA at any intensity, and decreased sedentary time, was associated with fewer peer problems in boys. For each additional hour in moderate PA, peer problems decreased by 0.84 points (p < 0.001). Relationships were broadly similar for emotional problems in boys. For girls, only light PA decreased peer problems (–0.12 points/hour, p = 0.037) and no associations were observed for emotional problems. Improved conduct and hyperactivity scores were associated with more sedentary time in active boys. Increased time at any intensity of PA was generally associated with higher conduct and hyperactivity problems in boys and hyperactivity in girls, although the relationship with vigorous activity appears to be mediated by self-esteem and cognitive ability in girls (p < 0.05). Both active and inactive girls had improved hyperactivity with increased sedentary time (p < 0.05).

Conclusion PA levels and sedentary behaviour affect particular aspects of mental health functioning differently in children. Associations vary by gender and, for sedentary time, whether the child met MVPA guidelines.