The rotator cuff is composed of four muscles

1. Supraspinatus – abduction
2. Infraspinatus – external rotation
3. Teres minor – external rotation
4. Subscapularis – internal rotation

Rotator cuff injuries are incredibly common and it is helpful to figure out which of the four muscles is the major source of the problem (there is of course the possibility that multiple ones are injured/irritated).

For the infraspinatus (innervated by the suprascapular nerve C5,6)  and the teres minor (innervated by the axillary nerve, C 5,6) you can test external rotation of the arm. The patient holds the arm next the body (adducted) with the elbow flexed to 90 degrees. The examiner then attempts to internally rotate the arm while the patient resists. Pain or weakness is a positive test.