Question: When hitting backhand in tennis, which shoulder muscle is primarily responsible for that movement?

Answer: The infraspinatus muscle. The backhand motion is a forceful lateral rotation of the humerus. The infraspinatus muscle lies in the infraspinatus fossa just below the spine of the scapula. At the lateral edge is the teres minor, which works along with the infraspinatus. The tendon of the infraspinatus is very wide and long.

We use the infraspinatus when we reach into the backseat of the car or when we pull a sweater over our head. If you are right-handed, you are using your infraspinatus as you write across a page. When you lift your arm to blow dry your hair, your infraspinatus is at work.

True or False? Resisted lateral rotation of the humerus primarily tests the supraspinatus muscle-tendon unit.

Answer: False. Lateral rotation of the humerus is the primary test for the infraspinatus muscle-tendon unit. When the infraspinatus is injured, the tendon is most commonly the structure at fault. Injury to the tendon can occur in the body of the tendon, but it is more often strained at the tenoperiosteal junction where the tendon attaches to the bone.