Well, it's never JUST your shoulders, it's never just anything.
Your shoulders connect down through your trunk, and above them lives your neck, and a series of important muscles called the Scalenes. They're a group of muscles that suspend your shoulder girdle off of your skull somewhat like a macrame plant holder from the 60's. Think of a series of ropes suspended off of a ring, and then add a strong diagonal cord that stabilizes/mobilizes diagonally front to back and a long flat band down the back that flanks a beaded rope and you've got a visual of what's going on up there, between your Scalenes, Sterno-Cloidal-Mastoid muscle and your Trapezius, which travels down your back in it's broad, flat, diamond shape.
Just like this plant holder, if any one of these cords gets twisted or knotted, the entire balance of this suspension mechanism goes off.
This little series helps to restore balance all the way around.
Carry your plants gracefully, my friends.