The infraspinatus muscle is one of four main muscles of the rotator cuff that stablizes the shoulder joint and its primary function is to externally rotate the upper arm. The infraspinatus muscle is best tested by externally rotating the upper arm with the elbows flexed at 90 degrees. The elbows can either be at the patient's side or abducted 90 degrees to further isolate the infraspinatus from the deltoid muscle. Remember to test active and passive range of motion, as well as strength against resistance!