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What is the “Rotator Cuff”? (and why does it cause shoulder pain?)

August 26, 2021

If you have shoulder pain, you’ve probably heard the term “rotator cuff” before, but you might not know what the rotator cuff actually is.

In this video, I’ll explain how the rotator cuff is made up of four small muscles, and I’ll tell you how these muscles function to stabilize your shoulder joint when you move your arm.

These muscles include the supraspinatus, infraspinatus, teres minor and subscapularis. 

Towards the end of the video, I’ll also explain why rotator cuff pain is so common (things like rotator cuff tendonitis and tendinosis, as well as impingement).

This should help you understand why shoulder pain is so common, and why the rotator cuff muscles are often involved.


– Luke Gordon, DPT (Doctor of Physical Therapy/Owner of Gordon Physical Therapy)

*** More detailed written information is featured BELOW the video… I hope it helps!

The rotator cuff refers to a group of four small muscles that function to stabilize your shoulder as you move your arm. 

While you don’t necessarily need to know the names of these four muscles (there’s no test, I promise!), here they are anyways:

Your 4 Rotator Cuff Muscles

  • Supraspinatus
  • Infraspinatus
  • Teres minor
  • Subscapularis

The easiest way to think of these muscles is to consider them to function as “stabilizer” muscles.

While your larger muscles (like your deltoids, your biceps, and your triceps) function to move your arm and perform activities that require strength and power, your rotator cuff muscles are constantly working to keep you shoulder joint in good alignment.  

Before I explain that in a little more detail, let me just clarify something: when I refer to your “shoulder joint”, I’m talking about the primary ball-and-socket joint, which is the gleno-humeral joint.  

When it comes to your shoulder mechanics, you have three other joints that are also involved. This includes your AC joint, your SC joint and your scapulo-thoracic joint. 

I don’t mean to confuse you with all that… I just wanted to make it clear that when I refer to your “shoulder joint”, I’m talking about your ball-and-socket joint. 

With that said, let’s talk about your gleno-humeral joint, which features a large ball and a relatively shallow socket. 

There are some good things and some bad things about this type of shallow ball-and-socket joint.  

The good thing is this: you have a lot of MOBILITY in your shoulder (at least when it isn’t hurting). 

It can move in three different planes of motion, and it allows you to perform a multitude of activities with your hand (like reaching, throwing and lifting activities). 

Your shoulder allows you to perform some fun activities too, like golf, pickleball and frisbee, just to name a few… that’s pretty nice!

But… this abundance of mobility comes with a cost. 

The bad thing about all this mobility is this: the cost of all this mobility is decreased STABILITY. 

To demonstrate this point, let’s compare your shoulder joint to your hip joint. 

Your hip joint is also a ball-and-socket joint, and it can also move in three planes of motion. 

However, your hip features a much deeper socket, which means that you have LESS mobility and MORE stability. 

This is why it’s so much more common to dislocate a shoulder than it is to dislocate a hip. 

So, in good times, when your shoulder doesn’t hurt, all that mobility is really nice to have. 

And since there isn’t much “bony/structural stability” within your shoulder, your body relies much more on “muscular stability” to keep your shoulder in good alignment.

And as you might be guessing, “muscular stability” is exactly what your rotator cuff is made for!  

While those large arm muscles are busy moving your arm around to perform various functional tasks, your rotator cuff muscles are constantly working to keep the joint in optimal alignment. 

That all sounds great, right? 

Except when it isn’t…

I mentioned in my free report how most people with shoulder pain report an “insidious onset” of pain. 

This is a fancy way of saying that there was no specific cause or injury that can explain the onset of pain. 

Now that we’ve covered what the rotator cuff is, let’s talk about the next logical question: what causes shoulder pain? 

To answer this question, let’s just talk about the most common type of shoulder pain, which is irritation of the rotator cuff. 

We’ll save some of the other diagnoses for another email, as I don’t want to overwhelm you.

As I just casually mentioned, irritation of the rotator cuff is the most common cause of shoulder pain. 

This is typically referred to as “rotator cuff tendonitis” (tendonitis = inflammation of a muscle tendon). 

Here’s how the rotator cuff gets irritated, especially the supraspinatus muscle:

  • Following repeated, normal use of the arm, the rotator cuff can get mildly irritated (just typical overuse)
  • Mild pain usually leads to a period of REST to allow the irritated region to heal
  • However, the rotator cuff muscles don’t heal very well on their own…
  • As the pain persists, the rotator cuff gradually becomes weaker due to decreased use
  • Rotator cuff weakness leads to additional pain, as the shoulder mechanics deteriorate 

There are some interesting points to dissect in that little string of events. 

->> First Key Point: while most muscles heal quickly following mild overuse/inflammation, the rotator cuff muscles do NOT heal quickly.

This is especially true for the supraspinatus muscle, which sits right on top of your shoulder joint. 

We’ll discuss this key point in more detail in another blog post.  

->> Second Key Point: when the rotator cuff muscles weaken over time, this actually makes the pain worse. 

This is a fairly unique feature of the shoulder and the rotator cuff, and it involves one of the key functions of the supraspinatus muscle. 

As you raise your arm (let’s say you’re raising it out towards your side), the ball portion of your joint is rolling UP towards the top of the socket. 

The only way for the joint to stay in good alignment is for the ball to also slide DOWN in the socket at the same time. 

Otherwise, the ball rolls to the top of the socket and gets stuck there. 

The supraspinatus muscle is responsible for getting the ball to slide down as it rolls up… so, when this muscle gets painful and weak, it can no longer help the ball slide down. 

And here’s the real kicker: if the ball doesn’t slide down in the socket, the supraspinatus will get PINCHED when the ball rolls up. 

And voila, you have even more shoulder pain!

Let’s recap that very quickly before I wrap up this blog post. 

Because of the unique anatomy of the rotator cuff, as well as its poor natural healing capacity, pain often leads to weakness, and weakness leads to even more pain.

This process happens gradually over time, typically a period of months, and then you realize that you have shoulder pain on a regular basis.

Does that sound like you?

If so, I have great news for you: this type of pain is very common, and we help our clients get rid of this pain every day at the clinic!

As you can imagine, it’s not always a “quick fix”…

Once we identify the root cause of your shoulder pain, we have to get to work navigating the “3 Phases of Healing”, which includes:

  • Pain reduction/tissue healing (Phase 1)
  • Restoration of strength and ROM (Phase 2)
  • Full functional use of the arm (Phase 3)

This process can take weeks to fully accomplish.

However, if you’ve been dealing with shoulder pain for months (maybe even years), it probably makes sense that a full recovery will take a bit of time. 

But again, the good news is this: it can be done! 

I hope today’s post has given you some good background information about shoulder pain and the rotator cuff.

If you have any specific questions about your shoulder pain, please feel free to shoot me a quick email at with some details about your situation. 

I respond to every email I receive, and I’m happy to help you in any way I can. 

Best of luck!

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