I'm 20, and fairly slim and happy with my figure - except for my bottom. It's not fat, just saggy. Can you recommend any exercises?

A pert bottom is determined by the effective recruitment of the gluteal muscles, which make up the shape of your posterior. The problem is that the hamstrings, the major muscles at the back of the thighs, work in conjunction with the gluteals to extend the leg behind you - and tend to be stronger than them. So the hamstrings become very toned (since they are quite happy to do the work for both muscle groups), leaving the gluteals saggy.

To find out if you have lazy gluteals, try this simple test; you'll need a friend or partner. Lie face down, legs outstretched, forehead resting on hands. Get your partner to place one hand palm down on the fleshy part of your bottom and the other on the mid part of your hamstring. Slowly lift each leg about eight inches (20cm) off the floor. Your partner needs to tell you whether the hamstrings or gluteals contract first.

For a toned bottom, the gluteals must contract first. If yours don't, continue this test to develop the gluteal memory pathway. One tip is to pre-lengthen the leg before you lift it. Once this is mastered, you should get better results from classic buttock exercises such as all-fours leg lifts. Effective gluteal contraction occurs here only once the knee comes higher than the level of the hip, while allowing the pelvis to move. Keep the torso stable through deep abdominal contraction, sucking the belly button to the spine while engaging the pelvic floor muscles.

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