Although ab exercises don't directly build bone like jumping and other weight bearing exercise, strong abs are key to good posture and can protect your back from injury.
A good exercise for strengthening the transverse abdominal muscle is to lie on your back, bring your knees up, with lower legs parallel to the floor, and slowly straighten each leg, one at a time.
Here's an example of leg lifts with a stability ball. As with any exercise, consult with your health care practitioner or personal trainer for advice. Technique, form and recommendations vary.