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.

When studying dead shoulders, most big shoulder muscles change what they do depending on how the arm is positioned — but one part, the bottom part of a muscle called subscapularis, always does the same job: helping the arm move inward and across the body, no matter the position.

In dead shoulders, two muscles help lift the arm but twist it in opposite ways—one turns it outward, the other inward—so they work together like a team that also pushes against each other to keep the shoulder steady.

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 in front of you, certain shoulder muscles are better at helping the movement because of how they're positioned, while others are stronger at pulling the arm back down.