The Target: Your hips, butt and hamstrings
Lie on your back on the floor and place your lower legs on a Swiss ball. Place your arms out to your sides at a 90-degree angle, your palms facing up [A]. Now push your hips up so that your body forms a straight line from shoulders to knees [B]. Without pausing, pull your heels toward you and roll the ball as close as possible to your butt [C]. Pause and reverse the motion by rolling the ball back until your body is in a straight line. Lower your hips back to the floor and repeat.
If that's too hard: Simply raise and lower your hips without performing the "leg curl" portion of the exercise (don't roll the ball toward your butt). Each time you raise your hips, pause for three to five seconds.