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.

When you lift your arm forward of the usual shoulder line, your upper trap muscle works about 6% less hard compared to lifting it along the normal shoulder path — at least in people with healthy shoulders.

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.