Infraspinatus and Teres Minor
These muscles get worked when you do your typical rotator cuff work – you know the ones where you stand with your arm bent at about 90 degrees with your elbow kinda tucked to your side while you fling your arm back and forth into external and internal rotation. This one gets butchered a lot. Because the infraspinatus and teres minor perform similar roles I am going to attack them at the same time.
Where Are They?
Find the spine of the scapula again by placing your left hand on your shoulder-blade/scapula and poke around until you find a thin bony part that sticks out and runs horizontally. Let your fingers slide off this spine towards the ground and you will now have your fingers on the infraspinatus – to double check externally rotate your arm to feel it contract.
The teres minor is just below and it doesn’t start on the medial boarder like the infraspinatus and supraspinatus do. To find its exact location palpate your fingers between the lateral boarder of the scapula and the greater tubercule of the humerus. Externally rotate your arm to be sure you have the teres minor. If you feel a muscle contract when you internally rotate your arm you probably have your fingers on the teres minor which is just below the teres minor.
What do they do?
These muscles make up the posterior cuff which enables them to produce external rotation which allows you as a thrower to get your arm into external rotation and just as importantly allows the greater tuberosity to go underneath the coracoarcromial arch which prevents subacromial impingement. They also provide compression to the glenohurmeral joint in order to resist both superior and anterior translation. The “translation” of that last sentence means that it stops the head of the humerus (the ball) from rubbing up against the top (superior) or front (anterior) of the glenoid fossa (the socket) (Sharkey et al. 1995).
What’s the Best Exercise – Zero or Ninety?
The two most common positions to work the external rotators are with your arm at your side (zero degrees of abduction) or up at shoulder height (90 degrees of abduction). Computer models predict that the infraspinatus is stronger at zero degrees of abduction allowing it to produce 909 Newtons (N) of force versus 723 N at 90 degrees. The teres minor on the other is much more constant producing 111N at 90 degrees and 159N at zero – looking back at the pictures of the individual muscles you can see why the bigger infraspinatus can put out a lot more power than its smaller neighbor to the south. (Hughes et al. 1996)
Mike Reinold’s, who both the physical therapist for the Boston Red Sox and an active researcher, decided to conduct a study to determine the best exercise for the infraspinatus and teres minor. Reinold and his team tested the many different exercises which have been reported to activate the teres minor and infraspinatus in various positions (side lying, standing, prone) to finally decide which one reigned supreme.
His study reported that the side lying external rotation with the arm at zero degrees of abduction did the best job. The infraspinatus and teres minor contracted at 62% and 67% of their maximal voluntary contraction respectively. Adding a towel between your rib cage and arm allows you perform the exercise with perfect technique since it provides immediate feedback. Reinold reported that the addition of the towel increased the effectiveness of this exercises by 20-25% as indicated by EMG feedback from both the infraspinatus and teres minor. (Reinold et al. 2004)
The version that is performed at 90 degrees of abduction was shown to increase the involvement of the deltoid and supraspinatus which isn’t good when you are trying to isolate the external rotators. I still believe that this exercise has some merit since it replicates the position of the arm during the throwing motion however it is imperative to perform it properly – I commonly see player’s shoulders shrug up their shoulders increasing the activity of not only their deltoids but the trapezius as well. If you are able to pull your shoulder blades down and back you can add this version to your arm care repertoire but if you could only pick one or if you are returning from an injury stick to the side lying ER with a towel, support your head and use a weight that you can handle for 10-15 reps.