Contrary to popular belief, you don't have to do hundreds of crunches or other ab exercises to build core strength. One to three sets of 10 to 25 repetitions should be enough, according to the American Council on Exercise. Once you can do more than your target number of repetitions, add resistance or move on to more difficult variations on the exercise. Doing tough exercises means you receive adequate return on your investment of time and effort. Adding a stability ball increases the difficulty level even more, forcing your core muscles to work harder against the ball's constant instability.
Crunches
According to a 2001 study commissioned by the American Council on Exercise, the stability ball crunch produces significantly more rectus abdominis and obliques activity than floor crunches. Sit on a stability ball, then walk your feet forward as you lie back. Stop when the ball contacts you from shoulders to hips. Squeeze your abs to lift your shoulders off the ball in a standard crunch motion, bringing your ribs closer to your hips. If you've mastered doing 25 stability ball crunches, bring your feet closer together; this makes balancing significantly harder. Once you've mastered that variation, hold a weight plate in both arms, extended straight over your chest as you crunch.
Pullovers
Regular pullovers done with a dumbbell, barbell or weight machine work every major muscle group in your torso. Stability ball pullovers do the same thing, but use your own body weight as resistance. The added difficulty of not just moving your body weight but stabilizing it against the ball's tendency to roll makes this a challenging exercise.
Kneel facing the stability ball. Place both hands on top of the ball, arms almost straight, thumbs pointing up and elbows pointing down. Lean forward, extending your body as straight as possible from knees to head as you reach forward, rolling your hands forward over the ball. Return to the start position by flexing your back muscles, pressing your arms straight down and in toward your body -- effectively rolling your hands back toward you on the ball and levering yourself back to the start position.
Pikes
Stability ball pikes unite almost every major muscle group in your body, including your back, chest, shoulders, arms, thighs and glutes. Just coordinating this many body parts working together is a challenge, and adding the wobbly ball qualifies this as a tough exercise. Drape yourself stomach-down over the ball. Walk yourself forward until you're balanced on your hands and the balls of your feet, which remain atop the ball. Your body should be straight from head to heels. Squeeze your abs to keep your spine flat as you lift your hips into the air, drawing the ball toward you with your toes. In the end position, your torso should be vertical -- head down, hips up -- and legs straight, and your toes should still rest on the ball. Ask a spotter to help you get into, and maintain proper position when you first attempt this exercise.
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