When you lift your arm out in front of you, certain shoulder muscles are better at helping the movement because of how they're positioned, while others are stronger at pulling the arm back down.

The top part of your chest muscle is really good at helping your arm move forward, while the middle part keeps your shoulder steady and the bottom part helps pull your arm backward — based on how these parts are shaped in dead bodies studied by scientists.

Different parts of big shoulder muscles actually work in their own unique ways because they pull with different strength and direction — kind of like how different sections of a fan blade might push air differently.

In dead bodies, the front and middle parts of the shoulder muscle are best at lifting the arm out to the side, while certain chest and back muscles are better at pulling the arm back down because of how they're positioned.

When your arm is resting down at your side, the supraspinatus muscle in your shoulder is in the best position to start lifting your arm out to the side—better than other nearby muscles—so doctors might use this position to test how well it’s working.

The upper part of your chest muscle doesn't help much when you push something overhead out to the side, because it's in a bad position to pull. But it works a lot better when you push something overhead in front of you, because then it's in a good position to help.