This muscle wraps around your rib cage and attaches to the bottom of your shoulder blades, and it looks like the serrated edges on a knife. It is an often overlooked shoulder blade muscle—it keeps your shoulder blades from winging up and out. Weighted Punches 1. Start lying on your back on the floor, a weight bench, or for an added core challenge, an exercise ball. 2. With a dumbbell in each hand, straighten both arms up towards the ceiling, open your chest wide, and let the back of your shoulders touch the supporting surface. 3. Then, keeping your elbow and wrist straight, "punch" up your right arm as if you were trying to push out your upper arm bone from your armpit, lifting just your right shoulder blade off the ball or bench underneath you.