Researchers in Croatia tracked over 300 children around age 10, measuring how well their hearts and lungs performed during a shuttle run test. They compared these fitness results against each child's daily screen time (phones, tablets, computers, TV) and overall physical activity levels.
The goal was simple: to understand whether screen time affected the developing fitness of primary school children, and whether this effect differed between boys and girls.
Boys' fitness drops with heavy screens, even if they exercise
Children who spent a lot of time on screens showed noticeably weaker heart and lung fitness than peers with low or moderate screen time. This pattern held true even after researchers accounted for how much physical activity the children did. A boy who plays sports for an hour but then spends three hours gaming will still have lower cardiorespiratory fitness than a boy who exercises the same amount but screens less.
The gender gap: why girls weren't affected the same way
The most striking finding was that this screen-time effect appeared almost entirely in boys. Girls showed no significant connection between how much they screened and their heart-lung fitness levels. Researchers couldn't explain why, but the pattern was consistent across the study. This suggests that screen time's physical impact on developing bodies may not be the same for all children.
Screen time and exercise work differently on the body
The study challenges a common parent assumption: that regular exercise cancels out screen time. The data suggests these are separate health behaviours. A child can be active and still suffer fitness penalties from prolonged sitting in front of screens. For primary school boys especially, the hours spent seated appear to matter independently of their activity levels. This is the age when heart and lung capacity is developing, making the timing particularly relevant.
Practical steps tied to these findings
- Set recreational screen limits (1–2 hours on school days) as a separate goal from exercise targets, not a reward for physical activity.
- Pay particular attention to your son's screen habits; this study shows boys may face greater fitness risks than girls from heavy use.
- Prioritise breaking up long screen sessions with movement breaks, since exercise alone doesn't offset the fitness effects of prolonged sitting.