The bridge off a gym ball is an excellent exercise to strengthen the hamstring muscles. To begin this exercise lie flat on the floor, with your heels resting on the gym ball and your arms by your side. Lift your bottom up towards the ceiling, using your heels to pull the ball towards you.
Imagine a line between your hips – which you try to keep level as you lift your bottom from the floor. As demonstrated, as you roll the ball towards you, try to keep a straight line between your shoulders, hips and knees. Only lift your bottom up as far as feels comfortable, to avoid causing strain on your hips or lower back.