Muscle Monday: Infraspinatus

I will be periodically releasing a blog post on Mondays, reviewing a particular muscle of the body.

Today we begin with a muscle known as Infraspinatus.

Background

Infraspinatus is a muscle located on the back of the shoulder. It is one of the 4 muscles that make up the rotator cuff (the other muscles of the rotator cuff are supraspinatus, teres minor, and subscapularis).

Origin: Infraspinous fossa of the scapula (the back of the shoulder blade)

Insertion: Greater tubercle of the humerus

Actions: Shoulder external rotation; maintenance of the humeral head in the glenoid fossa.

Practical Function

As a pure mover of the arm, the infraspinatus is a powerful external rotator of the arm: it acts to turn our arm to the side (as you would do when reaching to close the door from inside your car).

More importantly, however, as part of the rotator cuff the infraspinatus muscle functions to allow the ball-and-socket joint of the shoulder to elevate (raise up overhead) properly.

The infraspinatus functions to pull the head of the humerus (the ball) into the glenoid fossa (the socket) whenever we raise our arm. Once again, this allows smooth motion to occur at the shoulder joint whenever we raise our arm up (as we do hundreds of times per day).

Pathology

Like most all muscles, the importance of the infraspinatus can be appreciated whenever it becomes dysfunctional.

If the infraspinatus is weak or damaged, the humeral head will ‘ride high’ in the socket, leading to an inability to raise the arm properly. An individual with a dysfunctional infraspinatus will often be unable to raise their arm as high as their uninvolved side, or do so with greater difficulty.

If the infraspinatus is severely damaged, the ‘high riding’ humeral head can be seen on an X ray with the arm resting at the side. Note that the picture on the left shows good spacing between the bones, while the picture on the right shows a 'high riding' humeral head.

Finally, it is to be noted that the infraspinatus can itself be a source of pain, whether as a tendonitis (or better, tendinopathy) or through various degrees of tearing (note the tear in the picture below).

Training

Gratefully, like all skeletal muscles, the infraspinatus responds very well to resistance training and can certainly be strengthened, improving its function and often eliminating pain.

In my judgment and experience (and in agreement with the research literature), the best exercise for strengthening the infraspinatus is sidelying external rotation. Watch the video below on how to best perform this exercise.

Conclusion

The infraspinatus is an incredibly important muscle; without its proper function, we would not be able to raise our arm in an easy and pain free manner as we do hundreds of times per day. Unfortunately, many people fail to properly strengthen their infraspinatus and over time develop pain and difficulty because of this. Please don’t neglect this small but very important muscle!

If you are having shoulder pain, or would like assistance in developing an exercise program to keep yourself from every having it, please do not hesitate to contact me; I would be more than happy to help.