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.
If you have played baseball I can guarantee that you have done some form of shoulder exercises that are aimed at keeping your throwing arm healthy. These drills are now a staple of any good throwing program and have been integrated into the game of baseball. The problem that I see is that these exercises are so common that very little attention is given to which exercises we should be doing, how we should be doing them and with which tools (bands vs. weights). I see players today performing the same set of arm drills that I did back in the mid 90’s (I’m almost 30, yikes!!) yet there has been plenty of research performed in this area.
This is the first blog in a series that looks into how we can train the shoulder not only for injury prevention but for performance enhancement as well.
The Shoulder – Mobile Yet Hostile
The shoulder is delicate structure that literally hangs off the side of our body being held in place by a series of tendons, ligaments and muscles. Yet it can produce and harness extremely high forces. To give you an idea of the power that can occur at the shoulder you don’t have to look any further than research produced by Dr. Glen Fleisig that states the shoulder reaches rotational velocities of 7200 degrees per second during the acceleration phase. That is the equivalent of your arm doing 20 full revolutions in one second – that’s crazy!!
When you look closely at the shoulder joint, known in the medical community as the glenohumeral joint, you will discover that it is able to perform more movements with a bigger range of motion when compared to any other joints in the body. This is because has what is classified as a ball & socket joint and is relatively unrestricted by bulky muscles (the hip is also a ball and socket joint but doesn’t have the same range of motion)
The ball is the head of the humerus (the upper arm bone) and socket is the glenoid fossa/cavity which is part of the scapula (the shoulder-blade).
This mobility comes with a price and that is that is lacks stability. The shoulder couldn’t do many of its movements if it were bound up by surrounding muscles and bones which stresses the importance of strengthening the muscles that do stabilize the shoulder. This where the rotator cuff comes into play.
SITS down and learn
Anatomy professor’s commonly use the acronym “SITS” to help students memorize the names of the muscles that make up the rotator cuff.
S – Supraspinatus
I – Infraspinatus
T – Teres Minor
S – Subscapularis
Each of theses muscles performs its own role when individually fired however as a group their role is to dynamically stabilize the shoulder – this is the way that we use when we throw a baseball (Lee et al. 2000) When you do an overhead activity, like throwing a baseball, your deltoid muscle (a.k.a shoulder muscle) wants to pull the head of the humerus up and unless there it is counteracted by the muscles of the rotator cuff it will rub up against the top of socket, the acromion process (Brossmann et al. 1996) and potentially cause an impingement type of injury (Sharkey et al. 1995).
The Sweet Spot
Within the glenohumeral joint is an optimal spot where the ball (head of the humerus) rotates inside of the socket (glenoid fossa/cavity). I like to think of it as the “Sweet Spot” since every baseball player knows about this magical spot on a baseball bat where good things happen. The same is true with the shoulder joint – if we can train our rotator cuff to keep the head of the humerus in the “Sweet Spot” good things will happen and the chances of injuring our shoulder due to throwing go way down.
The next blog post will look at the each rotator cuff muscle individually and tell you what are the best exercises and how to do them properly.