Shoulder Impingement: Part 2 – What Happens at the Shoulder Joint During Impingement

So to recap on the last article, classic impingement of the shoulder is caused by excessive narrowing of the subacromial space, impinging on the structures contained within the space.  If you missed part 1, find it HERE.  Impingement can occur:

  • Under the coracoid
  • Under the acromion
  • Under the ligament that connects these two structures (Coracoacromial arch)

Impingement can also occur outside of the subacromial space in throwing athletes, where the supraspinatus and infraspinatus muscles of the rotator cuff impinge on the superior/posterior edge of the glenoid (The top/back part of the socket).   This is known as Internal Impingement and it occurs in this position below.

Is this even real?

Side Note: Does Internal Impingement occur during some exercises that require a lot of external rotation in crossfit?  Possibly, then again maybe not since this condition is specific to throwing athletes exposed to a large volume of throwing, changing the structure of the shoulder joint over time (In the same pitchers with internal impingement of their pitching arm, their non-throwing shoulder does not exhibit this impingement)  It does make me wonder when myself and many other athletes have a pinching pain in the back of the shoulder when in end range external rotation.  Shoulder pain during back squats anyone??

Ultimately, we have to aware of both types of impingement in order to keep our shoulders healthy while we exercise.

A subacromial impingement can be occurring because of a bony problem which is known as primary impingement.   It could also be caused by anterior/superior migration of the humeral head caused by weakness or muscle imbalance of the shoulder muscles.  The latter is known as secondary impingement. This is also where we can get impingement by either the acromion or coracoid.

In primary impingement, contents of the subacromial space are excessively impinged upon due to the shape of an individual’s acromion (4).  There are three different types of acromions:

As you can see, a type I acromion has much more space then a type III “hooked” acromion.  You can imagine how a type I acromion would have an easier time pressing weights overhead then a type III.  Also, studies in those with full thickness rotator cuff tears show that most of the time these individuals have a type III hooked acromion(2).

Unfortunately, there is nothing we can do short of surgery to correct a hooked acromion.  We can only really work in the confines of exercises that don’t cause pain.  In a study of 200 patients with various types of shoulder problems 18% had a type I acromion, 41% had a type II acromion and 41% had a type III hooked acromion (3).   So in people with shoulder problems, 41% may be shit out of luck.  This is a definite subset of people that will have to be careful about how they exercise their shoulders.

Side Note: Keep in mind that bones grow when they are stressed.  So this means that working through pain in the gym may be stressing that acromion to become even more hooked or to develop bone spurs in the surrounding areas.

In secondary impingement, due to either a muscular dysfunction or imbalance in the shoulder (4), the humeral head is not being held tightly in place with shoulder movement.  The rotator cuff and surrounding scapular musculature is not working properly.

If you don’t want the sciency explanation, just understand that the rotator cuff and scapular musculature needs to be strong and operating properly to keep the humerus centered in its socket with movement.   If the humeral head is not being centered in its socket by these muscles it will end up moving upward, impinging the contents of the subacromial space against the acromion or the coracoid.

Now, here’s the science.  If you don’t want it feel free to scroll ahead.

In the body, muscles fire together in order to create movement.  Raising my arm overhead requires multiple muscles firing at the right times with the right amount of force in order to produce normal movement.

Here’s where crap hits the fan in impingement.

Here is a cross section slice of your shoulder. “D” is the deltoid muscle of your arm.  “S” is the supraspinatus of your rotator cuff.  The white ball is the humeral head and it sits snugly in its socket to the right.  The red muscles end in white tendons that attach to the bone.  (JRF stands for joint reaction force, the arrow is demonstrating how the supraspinatus pulls the humeral head into the socket when operating properly)

Muscles function by shortening, so if the deltoid shortens or contracts its going to pull the arm overhead.  The supraspinatus also raises the arm up a bit because of where it attaches, but as you can see its major job when it contracts is going to be pulling the humeral head securely into the socket.   These two muscles working together is known as a force couple.

Notice how close the tendon of the supraspinatus is to the bone above it in the second picture.  Well this bone happens to be your acromion.  Between 70 and 120 degrees of abduction(raising your arm over your head)  this tendon is as close as it will get to the acromion.  Not a lot of room for error huh?  You can see in the above picture that if the supraspinatus is not firing properly the humerus will migrate upward and bang up against the acromion when you try to raise your arm overhead.

The force couple between the supraspinatus and deltoid can be thrown off if the supraspinatus is either:

  1. Firing later then it should
  2. Being overpowered by a stronger deltoid
  3. Weak and dysfunctional

These can all occur when the supraspinatus is weak or damaged.  Now, the subacromial space is small to begin with.  There is some degree of impingement that occurs every time you raise your arm overhead but this is normal.  Certain shoulder positions and posture, poor exercise technique, poor exercise balance and general overuse of your shoulder muscles (especially overhead) causes excessive impingement and can increase your risk of developing full blown impingement.

What’s helpful to understand is that in impingement all of the rotator cuff musculature is important and shouldn’t be neglected in an overall shoulder health program.  Each of the rotator cuff muscles contribute to depressing the humeral head to help prevent impingement.  With impingement, just like in the supraspinatus each of these rotator cuff muscles can become dysfunctional.

For instance, Sahrman has previously described that the subscapularis normally prevents the humeral head from moving anteriorly and superiorly(5).  As we learned earlier, humeral migration superiorly and anteriorly decreases the subacromial space.  A weak or long subscapularis has been cited as a precursor to impingement syndrome.

There are other players involved in impingement syndrome as well.  Weakness of the subscapularis is often accompanied by an overpowering and tight posterior deltoid muscle.  This is coupled with tightness in the posterior capsule of the shoulder.  Similar to the deltoid/supraspinatus force couple example this altered force couple and tightness will cause  superior anterior glide of the humeral head.   This all leads to a decreased subacromial space.   To make matters more complex, the muscles surrounding the shoulder blade also effect impingement, but we’ll give you a break for one week and discuss that in the next article.

That’s it for the complex stuff!  I wanted to put that information out there so that later when I make recommendations to prevent shoulder injuries, you’ll understand why.  For now sit tight.  Next week we’ll discuss the shoulder blade’s role in impingement.

Whew, I feel like that’s enough information for one day.   Thanks for reading guys, if you enjoyed this article please share it on Facebook and sign up for my newsletter on the top right hand part of my page.

Supraspinatus of Steel,

Dan Pope


  1. Behnke, R. S. (2006). Kinetic anatomy. (2 ed., pp. 35-56). Champaigne, IL: Human Kinetics.
  2. Bigliani LU, Morrison DS, April EW: The morphology of the acromion and its relationship to rotator cuff tears. Orthop Trans 10:228, 1986.
  3. Morrison DS, Bigliani LU: The clinical significance of variations in acromial morphology. Orthop Trans 11:234, 1987.
  4. Page, P., Frank, C. C., & Lardner, R. (2010). Assessment and treatment of muscle imbalances the janda approach. (pp. 195-207). Champaigne, IL: Human Kinetics.
  5. Sahrmann, S. A. (2002). Diagnosis and treatment of movement impairment syndromes. (1 ed., pp. 193-261). St. Louis, MO: Mosby.
  6. Wilk, K. E., Reinold, M. M., & Andrews, J. R. (2009). The Athlete’s Shoulder . (2 ed.). Philadelphia, PA: Churchhill Livingstone.

Leave a Comment:

Add Your Reply
FREE When you sign up - 6 Step Checklist: How to Avoid Injury and Train Pain Free for the Rest of Your Life