Swiss Ball Preacher Curl
STEP 1. Place a pair of dumbbells on the floor in front of a ball. Kneel directly behind the ball, and drape your arms over it to grab the dumbbells in an underhand grip. Let our weight move back toward your heels as you brace your triceps on the ball, forearms down.
STEP 2. Keep your back straight as you curl the weights up until your forearms are just short of perpendicular to the floor.