Which of the following is a typical symptom of rotator cuff injury?

Pain when lying on the affected side is a typical symptom of a rotator cuff injury because the rotator cuff is a group of muscles and tendons that stabilize the shoulder joint and allow for a range of shoulder movements. When an injury occurs to the rotator cuff, particularly tears or inflammation, the associated pain often becomes more pronounced with certain positions, notably when lying on the affected shoulder. This is due to the added pressure on the injury, which can exacerbate discomfort and lead to increased pain during rest or sleep.

The other symptoms mentioned in the options are not directly associated with rotator cuff injuries. Difficulty extending the elbow relates to issues concerning the elbow joint rather than the shoulder. Stiffness in the hip is indicative of hip joint problems, and frequent headaches can be due to various factors but are not a typical symptom of rotator cuff injuries. This specificity highlights the importance of recognizing the signs and symptoms that are directly associated with shoulder conditions, aiding in the accurate assessment and diagnosis of rotator cuff injuries.