Dumbbell External Rotation

Lie on your side with a light dumbbell in your top hand, elbow bent to 90 degrees and tucked against your ribs, forearm resting across your stomach. Keeping your elbow pinned to your side, rotate your forearm up and open like a door until it's roughly vertical. This is a small, slow movement; your shoulder blade and elbow shouldn't move, only your forearm rotating around that fixed elbow. Lower back down under control rather than letting the dumbbell drop, since the muscles here are small and easy to overload with too much weight.