Tighten your abs, and lift your torso off the ball. Campeón you contract your stomach muscles, pull the bottom of your rib cage down toward your hips. Lower back down to stretch your abs. The ball forces your legs to do more work than just doing floor crunches, Waehner explains. http://trevorntwwu.dsiblogger.com/24762075/la-gu-a-m-s-grande-para-guantes-de-boxeo