Another leg exercise for hamstrings, also developing core strength.

This exercise uses a fit-ball, also known as a gym-ball or swiss-ball.

Lie on your back on a gym mat with the fit-ball at your feet, positioned so that you can place your heels and Achilles tendons on the top of the ball. Lift your body into a "plank" position - straight and rigid from shoulders to heel - with your shoulders on the ground and your heels on the gym ball. Steady yourself with your arms but try not to take any of the weight on your arms.

Keeping your body rigid from your shoulders to your knees, bend your knees so that you draw your feet towards you, rolling the fit-ball towards you and raising your body, "hinging" on your shoulders. Your hamstring strength will be used to draw your feet towards you on the fit-ball as you raise your body. Your core strength will be used to keep your body straight as well as keeping the fit-ball controlled and preventing it from rolling to either side. Then slowly straighten your legs to let the fit-ball roll away from you as you lower your body back into the "plank", with your shoulders on the ground and your heels on the fit-ball.