My eldest son is overweight and had a bad experience with sport last year at school - he came home feeling very demoralised and embarrassed by what he perceived as his lack of physical ability. Any advice for this new academic year?

This is sad since a child's first exercise experience is important in forming their attitude to it. How their actions are encouraged will shape their impression of exercise. At home, always use positive reinforcement; well-intentioned suggestions such as, "If you don't ride your bike for 30 minutes, five times a week, you'll have to clean your room," are a big no-no. Instead, say, "If you ride your bike, you can go to the cinema."

While government guidelines advocate 60 minutes of activity a day, avoid getting hung up on the numbers; a study showed even 15 minutes boosted positive and decreased negative moods. Although children naturally love stop-start activities, they lack what is called metabolic specialisation (where their circulatory system has yet to develop to favour a particular form of exercise).

So it's good to expose your child to as many different activities as possible. This will assist him not only in finding more than one activity he can enjoy, but also provides more options to suit his metabolism as it matures. Weight-training can favour the heavier-built child and can be effective in weight management.

Using exercise as a form of punishment is one of the surest ways to turn a child off learning to love to move and should not be advocated by either parents or schools.

· Joanna Hall is a fitness expert (joannahall.com). Send your exercise questions to Weekend, 119 Farringdon Road, London EC1R 3ER (weekend@theguardian.com).