Shoulder Impingement
This video describes the relevant anatomy of shoulder impingement.
Shoulder impingement occurs when the supraspinatus muscle (one of the rotator cuff muscles) impinges underneath the shelf of the acromion process. The symptoms are often described as sharp when reaching overhead (elevation), across the chest (horizontal adduction) or tight when reaching behind the back (functional internal rotation).
The supraspinatus may become impinged for several reasons. The pinching often results due to an alteration in biomechanics of the shoulder. Muscle weakness or changes in the passive stability of the shoulder may cause or otherwise progress the underlying condition.
Treatment for shoulder impingement focuses on resolving the underlying cause. A proper examination and plan of care developed for your specific findings is often the best approach.