Infraspinatus tendinitis is one cause of pain in the shoulder.  The infraspinatus muscle is one of the muscles in the rotator cuff.  The belly of the muscle lies below the spine of the scapula and its tendon inserts on the back of the greater tuberosity of the humerus.  It is responsible for external rotation of the shoulder as well as elevation of the shoulder.  The pain of infraspinatus tendinitis is located in the back of the shoulder and the deltoid muscle as well.  The pain occurs when elevating the arm or rotating the shoulder backwards.  The pain may occasionally radiate down the arm and into the fingers.  Infraspinatus tendinitis is usually cause by excessive use either at the gym or by activities involving over the head motions.  The patient may also complain of a “catch” while elevating the shoulder.  If the disease is not treated, the range of motion of the shoulder is progressively diminished causing pain with everyday activities.  Sleep disturbance is also common.  X-rays and MRIs of the shoulder are useful to look for other causes of pain such as fractures or rotator cuff tears.  Blood work is useful to screen for autoimmune diseases.  Treatment begins with conservative modalities such as heat, ice, physical therapy and non-steroidal anti-inflammatory medications.  Injection into the area of the infraspinatus muscle and tendon with a local anesthetic and a steroid may decrease pain and allow more effective physical therapy.