Swiss Ball Hip Extension and Leg Curl

Lie on your back on the floor and place your lower legs on a Swiss ball.  
 
Push your hips up so that your body forms a straight line from your shoulders to your knees. 
 
Without allowing your hips to sag (keep with your body at all times), roll the ball as close as you can to your hips by pulling your heels toward you. 
 
To make it harder, perform the same movement with just one leg, holding the other leg in the air above your hips.