Crunch (on stability ball, arms straight)

Sit on the stability ball with your feet flat on the ground and your knees bent at a 90-degree angle. Lie back on the ball until your lower back is supported and your upper body is parallel to the floor. Place your hands behind your head or cross them over your chest. Engage your abs and lift your upper body off the ball, curling your shoulders towards your hips. Pause for a moment at the top, then slowly lower your upper body back down to the starting position. Repeat for the desired number of repetitions.