Sit on a Swiss ball with your feet flat on the floor, shoulder-width apart. Walk your feet forward as you lie back on the ball. Stop when the ball is under your hips, lower back, and middle back, knees bent 90 degrees.
Your lower back should feel like it's curved around the ball. Keep your head in line with your upper body. Place your hands behind your ears and draw in your abs.
Raise your chest up and slightly forward in a crunching motion. Do not pull on your neck to initiate the crunch. You've reached the end of your range of motion when the middle of your back loses contact with the ball. Pause, then slowly return to the starting position.