Hamstring Roll-ins

This exercise really targets the hamstrings and can provide a great workout for them.

1) Lie on your back with your feet on the ball.
2) Lift your hips off the ground, tighten your core muscles and keep this shape throughout.
3) Draw the ball in towards your glutes, bending your knees and using your hamstrings.
4) When the ball is as far in as it can go, push it back out away from you and straighten your legs.

Your arms can start off on the floor by your sides to provide some balance. When your core strength improves you may find you are able to place your arms across your chest. This gives you less contact with the floor and makes things harder.
Try and keep a constant hip height when doing this exercise. It is easy for the hips to drop when you straighten your legs back out.