@alFredo: Thanks for your question, it's a good one. A very common cause of pain at this site is known as supraspinatus impingement syndrome, and yes, it's an overuse injury. Basically, the supraspinatus, which is one of the four muscles of the rotator cuff, forms a tendon that crosses the top of the shoulder by running beneath the acromion process. When that tendon becomes inflamed for any reason, say after a rigorous weight-lifting session, it tends to swell, which causes it to rub against the process and essentially fray like a rope. If the tendon is not given a chance to heal properly, it can become a chronic condition--i.e. tendonitis--and the risk of a rotator cuff tear increases.