Side Crunch on a ball

Side crunches on the ball, also known as oblique crunches, are an effective ab exercise for increasing core strength. They activate both the abdominal and oblique muscles. Studies found that crunches performed on a stability ball (a.k.a. fitness ball or Balance Ball) boosted activation, or flexing, of abdominal muscles by 24 to 38 percent over crunches done without a ball.