These muscles originate on your shoulder blades then insert into your arm bones, or the humerus. The primary role of your teres minor muscle is to rotate your upper arm outward as if to catch a ball in your hand. The teres major helps draw your arm back toward the middle of your body and extend your upper arm behind you. 1. Hold a dumbbell in your right hand, then place your left knee and your left palm on a flat exercise bench. Your knee should be under your left hip, and your palm should be under your left shoulder. 2. Draw your belly button toward your spine to maintain a flat back throughout the exercise. 3. Extend your upper arm behind your rib cage as you squeeze your right shoulder blade toward your spine. 4. Hold the contraction for two seconds, then slowly lower the dumbbell until your arm is straight.