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  • Writer's pictureMia

What the hell do shoulders do? Part two.

In shoulders part one, I introduced some of the basic shoulder anatomy: three bones, a bunch of ligaments, some neat geometry, and a little bit of pressure physics. Today we're gonna talk about the physiology of the joints themselves. You'll have to wait til the next part to get hot and heavy with all the muscles that move them, because I talk too much to put that all into one giant blog.

The joints of the shoulder rotate like gears and the many muscles pull from every conceivable angle to make sure that you don't hurt yourself trying to wave down the Uber who's about to fly straight past your house. Why do drivers do that? There's literally a map on their screen and a voice telling them when to stop. And it's not like any of them is ever looking for a safe place to stop, considering the way most drivers seem to just...abandon driving when they feel they've made it. I've been in NYC waiting for a Lyft they call me agitated as fuck twenty minutes later and are like "I'm eight blocks away, can you walk to where I am?" Bastards.


There are two main movement considerations for when you're trying to understand how to get stronger shoulders/more mobile shoulders/injury-free shoulders.

1) How do the bones move around each other to provide support in every potential range of motion?

2) How do the muscles pull against each other to smoothly reach every potential range of motion?

If you understand these two things, you'll be well on your way to the boulder shoulders of your dreams. Remember, if you're not assessing, you're guessing. Understanding how movement is *supposed* to happen will make it much easier to actually make it happen.


How do the bones move?

I said in part one that your shoulder joint is made up of three bones: the collarbone, the arm bone, and the shoulder blade. These three bones essentially sit next to each other rather than within each other, which allows your shoulder to have more movement than any other joint in the body. However, even though the fit is loose, these three bones must be able to move wholly and squarely around their attachment points in order to allow the shoulder to move optimally. Any glitches at any of the joints can have major effects down the line. Imagine if your leadoff hitters got on base every time but then the third man in the lineup was doing this with every pitch:

The team will never be able to score with Fun Suzie over here just taking herself out every time. Same with your shoulder. If your AC joint is inflamed, neither your arm bone nor your collar bone will be able to move properly. Everything will depend on that AC joint getting back to normal.

So when it comes to the shoulder, how do each of these joints contribute to hitting some serious home runs as a team?

The Sternoclavicular Joint (the SC joint)

Believe it or not, shoulder movement starts dead smack in the middle of your chest, with the point where your collarbone meets your sternum.

Go ahead, sit up very tall and trace your collarbone with your fingers until you find the bumpy end of it at the base of your throat. If your left hand is on your right collarbone, lift your right arm up overhead and back down again. You can feel the end of the clavicle rotating against the sternum, and that's step one in our Learn To Move The Shoulders Good And Do Other Things Good Too analysis.

Why did I ask you to sit up tall? Because hunching changes mechanics. Try this: keep your left hand on your right collarbone and hunch your shoulders like a sullen fourteen year old boy with a bad haircut. Now staying hunched, do the same arm motion up overhead again. You'll probably feel your collarbone doing something completely different. That's no bueno. Look at these lovely celebrities:

You think Bieber is putting 225lbs overhead with that slouch? No way. As soon as he tries, he's gonna be like baby, baby, baby, ow. Because the forward slump from his shoulders and chicken head is gonna grind his collarbone to dust against the rigid sternum. Sitting up tall will pull the clavicles away from the sternum a bit and give them more space to rotate. Which is great, because remember that your collarbones act like a tent stake to give the rest of the shoulder some central support. Slouch = death to putting anything overhead.

The Acromioclavicular Joint (the AC joint)

Now keep your left hand on your right collarbone and trace it outwards towards your arm. As you get towards the pointy tip of your shoulder, you'll feel the clavicle flatten and then end near the tip of your shoulder. The very tip of your shoulder is actually the acromion process of the shoulder blade. (If you keep following that bone around, you'll feel it curve around and turn into the spine of the shoulder blade.) And thus, we have the acromioclavicular joint! Welcome! You've probably heard this abbreviated as the AC joint.

OK, preppie

The soft squishy region between these two bony ends is the AC joint, held together by several short, tight ligaments. It can't move independently, so it glides along at the mercy of other parts of your shoulder. But it does help to transmit forces out of the arm and into the thorax so that one day Bieber can put 225lbs overhead if he could just stop slouching.

Because the AC joint is at the mercy of other movements, it's a prime location for aggravation, arthritis, sprains, and tears. Shitty scapular mechanics can shove a lot of load and force up into that connection point at the tip of your shoulder. Your AC joint doesn't deserve that. It's one reason we need the humerus to move well in the glenoid fossa, giving us the...

Glenohumeral Joint (the GH joint)

This is the technical name for where the arm bone (humerus) rotates in the shoulder blade socket (glenoid cavity). This is a biggie. Every time you need to make a fast and dangerous one arm pirouette on the uneven bars, you need your glenohumeral joint to really help you sell it.

Queen Aliya Mustafina spins me right round.

The shoulder blade-arm bone connection point is extremely shallow (comparable to a golf ball sitting on a quarter), which is why the shoulder is so mobile and so unstable. There are a number of intelligent anatomical design features that help to keep your arm bone connected to your shoulder blade. We discussed a few of them already. But wait! Just like the wise men said after giving Jesus gold and frankincense, there's myrrh!!!!

One stabilizing feature we haven't covered yet is the labrum, which you have heard about from every miserable baseball pitcher and swimmer you've ever known. (Do you know a single former swimmer who isn't a rage-y stay-the-fuck-away-from-me adult? It's like participating in a sport which intentionally places you in a slow-moving, frigid medium where you can't breathe normally and other girls are assholes about your body automatically qualifies you to be a douchebag to everyone you ever encounter forever. And that's only describing the swimmers I like.)

The labrum is shown here, encircling the socket.

The labrum is sort of an anatomical washer, a fibrous ring around the glenoid cavity which is pyramidal: wider at the bottom where it attaches to bone, narrower at the top to allow for maximum movement of the humerus. Its location makes the socket just a little bit deeper to help add stability, but since it's flexible it doesn't compromise mobility. (If the socket were simply a deeper bone, your arm bone would be like your mom, slamming against the rim in every direction.) The labrum is strong AF because it has no blood supply to compromise the integrity of the cartilaginous fibers, but this also makes it a complete bitch to heal when it gets hurt. Although in fairness I am also a complete bitch when I get hurt.

Fun fact: a good part of your biceps muscle (the one you flex for pictures) attaches directly into the labrum. It's one reason that a biceps injury can feel like a shoulder problem, or vice versa. A great example of why professionals in person are better than the internet. Your symptoms may be deceiving you. Don't self-diagnose when you're in pain! Go medical-professional diagnose!

Scapulothoracic Joint

Technically not a joint the way we think of joints, but a vital part of the shoulder complex nonetheless. The shoulder blades swim in muscles and fascia, which help guide them up and around the rib cage. That's the scapulothoracic joint, where the shoulder blade meets the rib cage and moves along it. This juncture is the whole enchilada of the shoulder complex, because it dynamically stabilizes everything that helps your arm move in the socket. The largely uninhibited movement of the shoulder blade means that the AC joint, the SC joint, and the GH joint are all highly reliant on good scapular motion. And the opposite is true as well: if the Biebs doesn't stop slouching and cutting off rotation at his SC joint, the shoulder blade cannot move through its full range of motion either. How can his monkey sit properly on his shoulder if he's got a horribly inflamed supraspinatus tendon? Answer: He can't, and the monkey ends up in Germany.

We don't want your shoulder to end up like Bieber's. We want it to swim like a champion. We want it to swim though your back muscles like Michael Phelps in the 2016 Olympic finals.

Because you gotta understand. Your shoulder blade is a big ass bone. It doesn't swim in the local leagues. It's competing in the goddamn Olympic finals every day. It has a job. It has a responsibility. If you lost your shoulder blade, you would essentially only be able to move your arm from the elbow down. (Yes, you can lose your shoulder blade. Almost always it's from a very rare cancer. 1/10 do not recommend.)

The scapula is not some dainty little doily sitting underneath the champagne magnums of the delts, the lats, and the traps. No. The scapula stretches from your second rib (which attaches just beneath the base of your neck) all the way down to your seventh rib. It's about the size of a 12 oz. ribeye steak. For my vegetarians out there, that's a big ass steak.

And that big ass bone needs some big ass muscles to support and move it.

There are seventeen muscles that attach to the shoulder blade, including the crucial, indispensable, all-important rotator cuff muscles. We always hear about the rotator cuff because it's such a diva and demands endless attention, but the other thirteen muscles have their work cut out for them too. Together, all of these muscles pull on the shoulder blade from each of the compass directions, allowing the blade to move forward, backward, sideways, rotationally, and up down up down left right left right B A Start. (Where my CONTRA fans at?!)

Can you tell I'm writing this through self-isolation? Our poor brains are melting.

We're coming right back with part three to look at the muscle physiology and kinesiology that rules the roost, especially the rotator cuff muscles. See you then, you sexy muscular wildebeests.

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