Inflammation of the long biceps tendon is mostly due to prolonged overuse by repetitive overhead activities. It can also result from a sudden injury caused by an excessive force applied to the biceps tendons whilst lifting or carrying a heavy weight with the upper extremity. Additional factors can contribute to long biceps tendonitis:
Tear of the rotator cuff
Shoulder instability
Joint stiffness especially of the elbow
Lack of, or excessive training
Inadequate warm up
Weakness of the muscles
Tightness of the biceps