Rotator Cuff Subregions: Supraspinatus
Our previous deep dive into subregions of the shoulder didn’t yield a whole lot of ground breaking information. Consider it almost like a pilot episode for a new series; a bit shaky with half-baked ideas. This post is more like the season finale where they have earned your loyalty as a viewer.
Supraspinatus is a small, sliver of a muscle that resides above that boney ridge of your shoulder blade. It is generally known to initiate lifting your arm sideways away from your body, and helps to stabilize the shoulder. Despite being so small, the supraspinatus also has two distinct subregions that are critical to shoulder function; the anterior and posterior supraspinatus.
Interestingly, you can see actual structural differences between the anterior and posterior supraspinatus. This has helped researchers determine what their roles are and how they contribute to overall movement and stability in the shoulder.
To preface, I want to clarify some lingo here, notably the terms muscle and tendon. When we talk about a muscle, we usually mean the section of a muscle that contracts and does work. The tendon is the part that attaches the muscle belly to the bone to turn the contraction into movement. These connecting points are referred to as the musculotendinous and the osteotendinous junction (MTJ and OTJ). In textbooks, the muscle is typically coloured in red, with the tendon shown in a white tone. Not all tendons are the same - they can differ in size, structure, and stiffness, based on their purpose and how they are loaded. You can also have different ratios of muscle to tendon. Your biceps for instance has a long slender tendon that loops up towards the shoulder, while on the opposite end of the muscle, the tendon is stubbier.
Now that we have cleared that up, we can move on to the meat and potatoes of this post…
The anterior supraspinatus has a large, more tubular or rope like tendon, which allows it to function as mover of the shoulder. For example, if you had a paddle ball, a thin string would allow the ball to bounce around in all sorts of direction. Its activation varies based on load, elevation, and plane of movement. The higher the arm in raised, and the more to the side of the body, the greater the anterior supraspinatus has to work. It also contributes to both outward and inward rotation of the shoulder.
The posterior supraspinatus on the other hand has a flatter, strap like tendon, which contributes more to the stability of the shoulder rather than movement. If we go back to the paddle ball analogy, if you replace the string with a wide, flat rope, the ball would not be able to bounce around as much. It has also been shown to have a more consistent activation profile in different arm positions. If you wanted something to help provide security, you want that muscle to be working all the time.
Speaking of helping, remember how in my previous post I talked about the superior infraspinatus sharing a blended tendon insertion with the posterior supraspinatus? The body is filled with redundancies and this is a great example of that. Particularly in overhead positions, the posterior supraspinatus gets some stabilizing assistance from the superior infraspinatus. This means that if you were to partially tear one of those muscles, the other one that is intact would be able to take on more of that role. We see in clinic patients who have torn their supraspinatus tendon and are totally functional with no weakness or instability in the shoulder. This is likely due to the superior infraspinatus picking up the slack.
If you made it this far, congratulations! That’s a lot of reading. Here are 3 key takeaway from the word vomit above:
The supraspinatus works harder the higher the arm is, notably when it is in the scapular plane which is slightly in front of you (see the YouTube video below from UPMC), and when you bring the arm out to the side like during a lateral raise.
The anterior supraspinatus is a general mover of the shoulder.
The posterior supraspinatus acts as a stabilizer along with the superior infraspinatus, meaning that if you tear one of those muscles, you can strengthen the other to help take up the slack.
What does this mean for rehab?
Start down low, with your arm in front of you to start engaging the supraspinatus in a non-pain provoking way. Again, you can use isometrics, heavy lifts, etc to engage the muscle during the early stages of rehab.
The movements that most engaged the supraspinatus was doing a lateral shoulder raise and a shoulder raise 30 degrees or so in front of your body. Save these for more advanced rehab.
If you have torn your posterior supraspinatus, you can focus on rehabbing the superior infraspinatus so that it can pick up the slack. Refer to the previous post for some ideas!
Below is a video with some good ideas for strengthening the supraspinatus, keeping in mind how elevation and position affect the stress put on that muscle. As usual, if you are having challenges with these exercises, book an appointment with your preferred health care provider to get assessed and be given exercises specific to your condition!
