The Biceps is an upper arm muscle that acts across the shoulder and elbow joints. At the elbow, it helps to bend the joint. At the shoulder, its function is more complex and incompletely understood.
The biceps has two tendons around the shoulder: the long tendon passes inside the joint via a groove in the ball and attaches to the upper end of the shouldersocket, and the short tendon passes outside the joint to an adjacent bony outgrowth (coracoid).
The long tendon of the biceps is the problematic entity in shoulder pain.
Arthroscopic view (left) and diagrammatic representation (right) of the biceps tendon passing through the groove.