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, says Michele Olson, Ph.D., creator of the Perfect Legs, Glutes, and Abs DVD. "It is an often overlooked posture and shoulder blade muscle—it keeps your shoulder blades from winging up and out and does a lot to stabilize your shoulder girdle."
Best Exercise: Weighted Punches
Start lying on your back on the floor, a weight bench, or for an added core challenge, an exercise ball. With a dumbbell (Olson recommends using between 5-10 lbs) 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. 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. Alternate punching on both sides and perform three sets of 15 reps.