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.

When you lift weights overhead to the sides, your shoulder side muscles (medial delts) work harder because of how your arms move and your shoulders are built.

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.

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.

If you lift your arms out to the side at slightly different angles, it changes how much your shoulder muscles have to work — even small shifts can make one part of the shoulder muscle work harder or easier.