Devotees say it is the most effective way to trim waists and thighs and get a washboard stomach. That's not all. "When you circle a hoop around the waist, you need to switch on all your muscles," says Lorna Malcolm, director of the Reebok Sports Club near Canary Wharf. "Although the main movement is in the trunk and abdominal area, your legs and arms are working to stabilise you. Because you are using upper and lower body, it is a cardiovascular activity that will leave you out of breath."