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 healthy people lift their arms out to the side, their shoulder muscles work in a very similar way no matter if they're lifting straight out to the side, slightly forward, or in line with the shoulder blade — their muscle patterns stay consistent.

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.

People who play tennis don't use the shoulder blade muscles as much as other athletes when doing a specific exercise where they lie face down and pull their shoulder blades together, especially when their arms are raised high.

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

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.