The muscle, tendon and attached bone comprise a unit. The unit stabilizes the elbow and shoulder joints and allows their motion. A strain occurs at the weakest part of a unit. 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. Humerus and bones of the shoulder. Pain when moving or stretching the biceps muscle. Swelling around the injury.