My artwork

Hey there! You're not connecting the arm and the muscles correctly. You've drawn the deltoid as if it were in a neutral position, while the deltoid actually follows the movement of the clavicle and shoulder blade. Now, try imitating the pose yourself and pay attention to how your body moves to achieve it. To push the arm back like that, two main things are happening: first, there's the movement of the clavicle and shoulder blade—the shoulder blade retracts along the back (moving closer to the spine, so to speak). Second, the upper torso creates a slight twist; it doesn't remain completely straight or aligned with the pelvis. :) This is what helps create that movement. While I didn't exaggerate the pose too much in my correction, I drew it this way to show you how the chest muscle stretches along with the movement of the arm, and how the deltoid connects to the clavicle. I admit it's probably not entirely correct, and I'm sure I made a few mistakes, but I hope it still works as a helpful example. :)