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 you reach your arm forward, the back part of your shoulder muscle might actually work against lifting your arm up — helping instead to control rotation, not movement, at least in dead bodies studied in labs.

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

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.

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.

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.