When they refuse to get up from the sofa, it is easy to dismiss them as being lazy. But perhaps couch potato teenagers can’t help it. Experts say that there may be something about the biology of puberty that zaps youngsters’ energy and leaves them less interested in sport and exercise than before.
The theory comes from Exeter University medical school researcher Brad Metcalf who analysed data on almost 300 British schoolchildren who were tracked from the ages of five to 15. The data included information on exercise levels collected once a year when the boys and girls wore a movement-measuring device attached to their waist.
As in previous studies, the girls were found to do less exercise than the boys. The analysis showed that activity levels started to fall from the age of 9 – when some of the children would have started to go through puberty.
The European Congress on Obesity also heard that the drop in levels was sharper among the girls than the boys. Crucially, the children were as likely to opt out of exercise during school hours as at home.
This is important, as if adolescents were purely being lazy, you’d expect exercise levels to drop most sharply during evenings and weekends, when they are free from the structure of the school day. The analysis also showed that the transition from primary to secondary school had little effect on the amount of exercise done.
And, with activity levels starting to fall with the onset of puberty, Mr Metcalf said that couch potato teenagers may not simply be being lazy. Instead, there may be something about their growing bodies that is causing them to become less active.
He said: ‘There is enough consistency in our results to support a biological or physiological driver. ‘It isn’t definitive but it is about making people aware that it is possible that teenagers are not inactive because they are lazy, although some might be.’
Mr Metcalf isn’t sure what the biological process is but said it’s possible that the adolescent body conserves energy for growth and making muscle. In girls’ case, energy may also be needed to prepare the body for future pregnancies.
He added: ‘The findings may help explain why it has been difficult to increase the activity of children. ‘I’m not saying that you can’t but it means that maybe we need to design future interventions with this in mind.’
Tam Fry, of the Child Growth Foundation, described the results as ‘striking’ and said that although the explanation might be speculative, it should be not be dismissed. He added: ‘The fact that both boys and girls are tracking downwards in their physical activity is likely to have much more to do with evolution than laziness.
‘For millennia the body clock has been ticking towards the need to conserve energy for puberty and reproduction and this simple biological fact may dull the will to go for a walk if taking exercise is not a habit. ‘One can only hope that, once puberty is over, evolution will remind them as young adults to get up and running.’ (DM)