Dr. Jared Powell:
See if you recognize the following narrative. The supraspinatus muscle initiates the first 15 degrees of shoulder abduction, then the deltoid takes over. We need to strengthen your supraspinatus, so empty can exercise. Stop. Make it the full can exercise. It's less painful. Three sets of 10, thumb up. Don't let that shoulder shrug. Now, if you went through a physio degree anywhere in the world, I dare say you were taught that particular sequence.
Dr. Jared Powell:
And if you're someone with shoulder pain, you may have even been prescribed it. Now, I've got a few problems with this narrative. The assumption is that the supraspinatus is a discreet, identifiable, isolatable actor in shoulder function, that it has its own job and that it does that job first. And that we can access it with the right exercise and load it specifically. Today, we investigate that assumption. This is another episode of the Zombie Hunt series where we identify ideas that are wrongfully still rife within musculoskeletal rehab and hopefully put them to bed. I'm Dr. Jared Powell, and welcome back to the Shoulder Physio Podcast. Today's episode is all about the rockstar of the shoulder, the supraspinatus. Let's rip in. Okay, before I start knocking things over and making an awful mess, I want to build the strongest possible version of the argument in the firing line because it is the fair thing to do.
Dr. Jared Powell:
The story of the supraspinatus was assembled by careful people from good evidence. And if you'd been knocking around in 1980, you would have believed it too. I know that I certainly would have, and I did believe it for the first several years of my career, shamefully. So the case for the defense before the prosecution is as follows. Let's start with the anatomy and try and picture it as I describe it. You have your arm hanging by your side. The supraspinatus runs from the top of your scapula across to the greater tuberosity of your humerus. So its line of pull at that moment is almost horizontal. The deltoid at exactly the same moment is pulling almost straight up. Its line of pull, , directs the, the humerus straight north, which means that if the deltoid fires on its own from zero degrees of shoulder abduction, a big chunk of that force isn't rotating anything.
Dr. Jared Powell:
It's simply driving the humeral head north into the acromion. The supraspinatus, which pulls horizontally, pulls it into the glenoid and holds it there instead. That's a very neat force couple. One muscle compresses and centers, that's the supraspinatus, and the other elevates, that's the deltoid. And between them, you get this clean rotation instead of the humeral head simply riding up. It is a match made in heaven. Cadaveric work does show the rotator cuff opposes superior translation of the humeral head. That's not up for debate. Four studies do show supraspinatus exerting a medial compressive force at the joint. Take the rotator cuff out of a cadaver and glenohumeral mechanics measurably degrade. Moment arm studies show supraspinatus is well placed to produce abduction torque. Every single one of those points I just made is valid and this is not where I see the problem. Let us now introduce the clinical component to this concept.
Dr. Jared Powell:
The supraspinatus tendon sits in the most awkward real estate in the body between a moving ball, the head of the humerus and a fixed bony roof. It's the rotator cuff tendon we often injure the most. People with symptomatic tears are often painful and weak into shoulder abduction. It really does look and feel like the supraspinatus is the weak link in the chain. And then there's a logic of rehab itself, which I want to be clear about. This logic is very reasonable. If a specific tendon is the injured tissue, then progressively loading that specific tissue is a more than reasonable thing to want to do. If it's good enough for the Achilles, why not the supraspinatus? In 1982, two researchers describe a position designed to bias the supraspinatus and the empty can enters the world. People notice it's provocative and painful shoulders, so the full can arrives as the kinder variation.
Dr. Jared Powell:
Around the same time, this idea called concavity compression becomes the standard explanation for the rotator cuff as a whole. All four muscles of the rotator cuff squeezing together more or less equally, pressing the ball into the socket so the big torque producers like the deltoid have something stable to work against. Put it together and you've got a pretty universal theory of rotator cuff function. It's very elegant and it's almost entirely wrong. Go back through everything I just listed. None of that evidence comes from actually watching muscles inside living people while they move. That feels like a fairly significant absence of evidence. So let's go and find that evidence and see if it changes our narrative. Okay, so let's start with the claim perpetuated in anatomy textbooks on every single student's bookshelf, including my own back in the day. Supraspinatus initiates shoulder abduction. Okay, so first question, what would have to be true for that particular claim the supraspinatus initiates shoulder abduction to mean anything? So a group out of Sydney working in Karen Jin's lab, and I'm gonna mention Karen Jin a lot in this episode because she's the godmother of this research, laid this out about 10 years ago. If you want to call a muscle the initiator of movement, two things have to be true. Okay, these are the conditions that need to be satisfied.
Dr. Jared Powell:
It has to switch on before the movement starts, obviously, and it has to switch on before the other muscles move that joint. Those two criteria need to be satisfied to say that any muscle initiates any movement. I think the second one is the most important one, that it has to switch on before other muscles that move a joint. If everything comes on together, then nothing initiated anything. They all started the movement together in unison. So that's the bar. Up until 10 years ago, roughly, nobody had tested whether the supraspinatus meets this bar for initiating abduction. What we did have though, was a boatload of disconfirming evidence going back many decades. Let's go all the way back to 1963. Researchers paralyzed the supraspinatus in living human beings and those human beings abduct their shoulders just fine. In 1986, the same finding. In 1996, a cadaveric model, same result.
Dr. Jared Powell:
2007, someone blocks the suprascapular nerve and finds the deltoid simply just increases its own activity to compensate. Supraspinatus goes. Deltoid just increases its own activity to compensate. The arm still goes up, the arm still abducts. So 60 years of disconfirming evidence, 60 years of people figuratively switching the supraspinatus off and still watching shoulders magically abduct. You'd think this would prompt an inquiry into the purported importance of the supraspinatus in shoulder abduction. It did not until the Sydney group in Karen Jean's lab. So they took 14 people with no shoulder pain in the previous couple of years and had normal full range of motion. Seven muscles were measured with fine wire electrodes into the deep muscles. Supraspinatus, infraspinatus, subscapularis, serratus anterior, lower trapezius. Surface electrodes were only used for the middle deltoid and upper trap. They even used real-time ultrasound to guide the needle into the lower trapezius because it's thin and it's easy to end up somewhere else entirely.
Dr. Jared Powell:
So the methods were sound. Then they instructed the participants to abduct into three different planes with three different loads, a sensor on the wrist to catch the exact instant the arm starts moving. And instead of eyeballing anything, they used an algorithm to detect muscle onset, the first 25 millisecond window where activity exceeds three standard deviations above baseline. So what did they find? Now you know the intricacies of the experiment. They found that every single muscle switched on before the arm moved. All seven muscles were switched on before the shoulder abducted. And between the supraspinatus, the infraspinatus, middle deltoid, upper trap, lower trap, and psoriatis anterior, no statistically significant difference in onset at all. All six of those muscles coming together, activating before anything moves at all. One muscle did stand out though and that, but that was the subscapularis and it stood out by being late.
Dr. Jared Powell:
It came on after all the other muscles had activated, which kind of makes sense. So their conclusion, and I'll read this verbatim so as not to make any mistakes, it is therefore misleading to refer to supraspinatus as the initiator of shoulder abduction. And then this doozy of a quote, which I love, "Anatomy textbooks need to reflect the fact that shoulder abduction is not initiated by supraspinatus alone, but by a group of shoulder muscles in a complex coordinated manner." Let's be precise and try not to fall victim to the attention economy. There's enough of that in podcast land without me following suit. So supraspinatus is active during abduction. That is not disputed. We've known this for the last hundred years roughly. It has an abduction moment arm. It does contribute torque. That's all fine. What theoretically died in this paper was the idea that it's temporally special.
Dr. Jared Powell:
When I say temporal, I mean that it comes on before others, that it's the trigger of abduction, that there's a first 15 degree window or whatever it is that belongs exclusively to the supraspinatus muscle and that it hands over to the deltoid. There isn't, that sequence doesn't really exist. It shows up at exactly the same time as every other m- muscle that was measured except for the subscapularis. So maybe you're happy to concede that the supraspinatus may not have a special role in shoulder abduction, but surely the exercise part holds up. The rehab part holds up. Surely the empty can and the full can and all those muscles that isolate the supraspinatus, , aren't in dispute here. Surely they are still valid. Surely we can get at the supraspinatus with very particular exercises. There's a study from 2009 that actually tested this, so let's have a look.
Dr. Jared Powell:
And again, no coincidence, it's come out of that Sydney lab again led by Karen Jin. They started by taking seriously the criterion that the field had set for itself. The ideal supraspinatus exercise, in the words of the researchers who framed it, should elicit the greatest amount of supraspinatus activity while minimizing the surrounding muscular activity, particularly the deltoid. So this is a really good testable definition. The best supraspinatus exercise should, in theory, be the one that elicits the greatest amount of supraspinatus activity while minimizing surrounding muscle activity. Good testable definition. So they took 15 people with normal shoulders. Again, they used fine wire into supraspinatus and infraspinatus and used surface electrodes over all three heads of the deltoid. Then maximal isometric contractions in the five positions of literature had crowned as the best supraspinatus recruiters. They were full can, empty can, prone elevation, external rotation with the arm by your side, and prone external rotation.
Dr. Jared Powell:
Finding number one, there was no significant difference in supraspinatus activation across any of those five positions that I just described. Every position lit up equally. The whole empty can versus full can debate turns out to be entirely meaningless. Finding number two. During both the full can and the empty can, the activation of all five muscles they measured was not statistically different. So the exercise invented specifically to separate the supraspinatus from the deltoid turns out to activate the deltoid every bit as hard as it activates the supraspinatus. Finding number three is pretty funny. The two positions that came closest to satisfying the criterion, good supraspinatus activity with anterior and middle deltoid quieter than the empty and full cans produced were the two external rotation exercises. So by the field's own definition, the optimal supraspinatus exercise is simply an external rotation exercise, except in both of those external rotation exercises, the infraspinatus was activated significantly more than the supraspinatus.
Dr. Jared Powell:
So what we have here is that the best supraspinatus exercise we have is actually an infraspinatus exercise. But there's a caveat. These have all so far been healthy shoulders. Everything I've described so far was measured in young, healthy, pain-free shoulders. And maybe in a painful shoulder, something does change. Maybe the supraspinatus does drop out or violate or get inhibited or something. So has anyone explored this? Turns out, yes. Rita Kinzelar and Tanya Pizari out of La Trobe in Melbourne did a literature review and found 22 studies that directly compared EMG between people with rotator cuff related shoulder pain and people without. So in the end, they had 342 people with painful shoulders and 334 with pain-free shoulders. Five studies looked at the supraspinatus specifically and across all of those studies, regardless of task, regardless of load, regardless of arm position, there were no significant differences between the painful shoulders and the healthy ones.
Dr. Jared Powell:
So the authors concluded for the majority of muscles, regardless of task, load, or arm position, significant differences were either not demonstrated or the results contradicted each other. Now, it would be remiss not to talk about the limitations of EMG here because there are numerous. The biggest one is that EMG data relates to muscle activation and is not necessarily directly correlated with muscle force or torque production. So keep all of that in mind, please. Okay, so what is the rotator cuff doing? Let's go to Karen Jean's lab again, this time from all the way back in 2011. They found that the rotator cuff displayed preferential activation in certain positions. So they found that during shoulder flexion, the supraspinatus and infraspinatus are more active than during shoulder extension. Conversely, during shoulder extension, the subscapularis were significantly more active than it was during shoulder flexion. I think this is a pretty cool observation with consequences.
Dr. Jared Powell:
So the rotator cuff is already doing its job during ordinary movement. You don't have to go looking for rotator cuff activation with special or magical exercises. Flexion will load the rotator cuff. Extension will load the rotator cuff. It's not a mysterious set of muscles that we must constantly reinvent the wheel to target it. Just move the shoulder. If you wanna preferentially activate the posterior cuff, flex the shoulder if you want to preferentially activate the anterior rotator cuff, the subscapularis, extend the shoulder. If you want to activate all of it, abduct the shoulder. Pretty simple. From the same paper, the pattern held at low, medium, and high load. So when they increased loads, everything just got louder, the muscles just activated more. Nothing changed in terms of sequences or patterning of activation. More load just meant more activity. Their own practical implications section of the paper makes the point better than I can and I'll read it.
Dr. Jared Powell:
Flexion and extension recruit the cuff and stabilizes while rotation exercises recruit it as a torque producer. So despite a lot of flack that shoulder rotation exercises get on social media, a simple, boring, old, external rotation exercise is still a really good exercise to activate or load the rotator cuff. And I must admit, in my practice, it forms a staple in much of my rehabilitation. In our rush to just get people back to bench pressing and lap pull downs and shoulder presses, which I'm an advocate for, there is still a role for simple, boring, old, external rotation exercises. So let's begin to summarize all this. What are the take home messages? First, I think we should stop saying that the supraspinatus initiates shoulder abduction. It doesn't initiate shoulder abduction. It is active during abduction, but it works with a number of other muscles. It doesn't appear to have a special role.
Dr. Jared Powell:
Look, the supraspinatus is still absolutely important, and I have to qualify that. It's obviously not a vestigial muscle. I just think that we have attached far too much importance to its role in initiating abduction. That's all. That is the topic and the thesis of this podcast episode. I want the supraspinatus to be as healthy as possible in all individuals. Second, let's stop selling rehab exercises as muscle specific. There's probably no such thing as a supraspinatus exercise. The empty can activates your deltoid just as hard. The best supraspinatus exercise activates the infraspinatus more. That's fine. Patients really only care about moving their shoulder again and getting back to their valued activity. They don't care about which muscles are moving their shoulder, and nor should you. Third, our physical assessment tests can't really isolate the supraspinatus muscle tendon complex, so we should stop saying they do. Fourth, we should care more about movements than muscles in rehab.
Dr. Jared Powell:
I know I just alluded to this, but let me expand on it. If abduction hurts or external rotation hurts, that is what's important. We shouldn't worry about distilling that to, well, if abduction hurts, that must mean that this muscle or tendon is specifically and uniquely compromised. Therefore, me, we must do this exercise to get at that particular muscle or tendon specifically. Try and figure out the constraints of the movement first. Why do you think it's hurting in this position? Is it a load issue? Can you modify that position to make it less painful? Do you need to temporarily unload the movement and then reload it? Are you suspicious of a compromised rotator cuff that needs surgical referral? Are you even sure it's a rotator cuff issue in the first place? So on and so forth. And fifth, change the conversation you're having with the person in front of you.
Dr. Jared Powell:
I think we need to move away from, we need to strengthen your supraspinatus, which is a story about a small broken part of your body. Two, we're going to increase the capacity of what you and your shoulder can tolerate in the directions and speeds you need to get you back to your life. And this is a story about a person relative to their goals and their values, and I think it's a much better story. So the zombie we came for today was the idea that the supraspinatus is a special muscle you can single out. It isn't, you can't. It's still a very cool and important muscle and I wish no ill will to the supraspinatus, but it works with other muscles of the shoulder complex to abduct the shoulder. Okay. Thanks for listening to this episode of the Shoulder Physio Podcast. I'm Dr. Jared Powell.
Dr. Jared Powell:
I'll chat to you soon. The Shoulder Physio Podcast would like to acknowledge that this episode was recorded from the lands of the Terribalange people. I also acknowledge the traditional custodians of the lands on which each of you are living, learning, and working from every day. I pay my respects to elders past, present, and emerging and celebrate the diversity of Aboriginal and Torres Strait Islander peoples and their ongoing cultures and connections to the lands and waters of Australia.
