Muscles, tendons and bones comprise units. These 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. Muscles and tendons that attach to bones in the shoulder. Bones in the shoulder area, including the humerus, scapula and clavicle. Pain when moving or stretching the shoulder.