Dynamic is the perfect way to describe this upper-body blast that works your triceps, chest, shoulders, and core. To do it: Start in a pushup position, with your hands on a BOSU trainer and your feet hip-width apart (a). Lower your left forearm onto the BOSU (b), then your right, keeping your body in a straight line (c). Push back to the starting position, starting with your left hand. That's one rep; do 10 to 15. Rest for 15 seconds, then repeat, leading with your right arm. Keep alternating for two or three sets.