Shoulder Impingement
This is the most common cause of pain in the 30 - 50 year old age group,but can occur outside this range also.
Impingement occurs when the ball of the head of humerus pinches the soft tissue between it and the arch above.
This may occur because (1) the head has went up due to loss of muscle strength holding it down (dynamic impingement), (2) the soft tissue between is larger than the space (due to swollen bursa), (3) swollen/torn rotator cuff muscle, (4) Acromio-clavicular joint arthritis (between collar bone and shoulder blade)