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.

When you lift your arm out to the side, the middle part of your shoulder muscle works best at the start, but the front part takes over more when your arm goes way up past shoulder height.

Rotating your shoulders outward when doing overhead presses makes the front shoulder muscle work harder than the side one.

The front part of your shoulder muscle is better at lifting your arm forward, while the side part works harder when you lift your arm backward — they’re specialized for different directions.

Lifting your arms out to the side or slightly backward, especially with your palms turned down, works your side shoulder muscles the most — this move might be the best way to build stronger shoulders.

The shoulder muscle called supraspinatus switches jobs depending on how you move your arm — it pulls one way when you lift your arm forward and the opposite way when you lift it backward, like a muscle with two different gears.