Muscles, tendons and bone comprise units. Units stabilize the shoulder and allows its motion. A strain occurs at a unit's weakest part. There is no loss of strength.
Strength is diminished.
Severe strain requires surgical repair. Chronic strains are caused by overuse. Acute strains are caused by direct injury or overstress. Tendons and muscles that attach the shoulder blade to the arm and chest wall. Pain when moving or stretching muscles of the shoulder blade.