Sunday, June 20, 2010

the amazing engineering that is the shoulder, part 1: scapula and shoulder girdle

How does the shoulder really work? What is it, anyway? When we think of working the shoulder, most of us likely think of the delts, maybe the traps (shoulder shrugs and all). When we think about shoulder injuries, the term "rotator cuff" enters the vocabulary and we worry about how to prehab/rehap these little stabilizer muscles. But really, the shoulder, or more particularly the shoulder joint and shoulder girdle is an amazing feat of evolutionary engineering, so much so that most of what we tend to think of as uppert body work is really shoulder work.

Consider that we usually think of the lats, traps, rhomboids all as back muscles, and the pecs and seratus anterior as chest muscles. Yes, ok, that's more about where those muscles are located, but they're all, really really, shoulder muscles. Even the triceps and biceps are involved as shoulder muscles - and require the shoulder as their anchoring points.

Understanding a little bit more about how the shoulder works in terms of where the muscles attach to what and how they act on the shoulder may help enhance our lifting, prevent injury, and help us understand what we're doing - or not - when we think about working out.

So the goal of this set of posts on the shoulder is to offer a wee tour through the shoulder, and provide further resources if you get fired up to look further.

Part 1: the Scapula and Shoulder Girdle

In structural kinesiology - the study of movement in terms of nerves, bones and muscles and joints - there are two main ways to look at the shoulder: the shoulder girdle and the shoulder joint. Both of these views considers one of the weirdest and coolest bones in the body, the shoulder blade or scapula.

In this first of two parts looking at the shoulder, we'll first take a quick look at the multifacetted structure of the scapula and also look at the muscles associated with the shoulder girdle - or the muscles primarily involved with moving the scapula itself.

In part two, we'll look at the muscles acting on the main shoulder joint, the glenohumeral joint (gh), from the rotator cuff muscles to the lats and pec major.

The Scapula: It's a wild wild bone.

This figure above shows three views of the scapula: the front side that faces the back of the ribs, the back side that we can feel or palpate, especially along that honking big spine, and where the magic focuses, the side on view that features the glenoid cavity - the aras where the arm - in particular the humerous fits into the shoulder.

Take a look at the ridges and bumps: every bit of an indentation or edge has a purpose in the muscular rigging that is the shoulder joint and shoulder girdle.

Overview: Let's take a quick look at the shoulder girdle muscles to get a sense of the specialness of the shoulder girdle design and what i mean by this rigging - and possibly why this approach in our design (is so cool).

The scapula first and foremost is situated behind and at the top of the rib cage. It has muscles attached to both sides of it: there are muscles along it's back; and muscles along its front. The ones we're looking at here are the ones that primarily move the scapula, and help keep it in position along the back of the rib cage.
As said, each bump, dip and pointy part of the scapula has a mechanical design purpose.  By way of example, the image to the left shows the back view of the scapula. In red, the levator scapula is attached to the superior border of the scapula. This shoulder muscle, while attached up at the top four vertebrae in the neck is not a neck muscle per se, but exists to help pull the scapula up (and a bit towards the spine, elevation and adduction)

If we look back at the image of the scapula bone, we see the medial border on the back/posterior side of the scap. Follow along to the medial border (left side of spine in the image with the levator scap shown) and the minor and major rhomboids are attached there and plug into the bottom of the cervical spine, and top half of the thoracic spine. If we just follow the line of the muscles, which go up diagonally, we can see that when they contract they'll pull the scapula towards the spine and up in elevation, but actually in doing so, can also rotate the shoulder socket down. We'll come back to why this rotation is important.

On the other side of the spine we see another rich and amazing attachment, the trapezius which is considered in three parts. The upper fibers attach along the far end of the clavicle or collar bone and the scapula's acromial process, and also along the start of the big spine on the back of the scapula. Again shoulder elevation helped here as well as adduction. Then the middle fibers which attach a little along the scapula spine, but also this time, rotate the socket up. The lower fibers of this massive muscle which connect on the scapula spine under side-ish area. These also contribute to pulling the shoulder socket up, but also bringing the shoulder down blade down.

To take a quick look at the front-ish part of the scapula, there's the pec minor and the seratus anterior.

The pec minor attaches to the scapula at the coricoid process (the biceps at one point do, too, among others) and then into the ribs. Again, if we follow the line of the muscles, we can see that when contracted, these muscles will pull the scapula, well, rather over the shoulder, rotating the socket downwards. The scapula also gets pulled away from the spine (abducted), and likewise dowward (depression).

The serratus anterior is an amazing set of muscles that connects all along the medial border on the inside/front of the scapula, on the opposite side from the rhomboids.

Where the rhomboids pulls the scap up and towards the spine, the seratus anterior pulls the scapula away from the spine and the scapula up at the same time via the attachment upwards along the sides of the ribs. The joint tension of the rhomboids and the serratus anterior both help keep the scapula down against the ribs in movements like the push up.

Why all this Scapular Movement? RANGE EXTENSION. I dunno about you, but while i've heard about shoulder depression and elevation and rotation, it hasn't meant a whole lot to me until i got to see what the bones actually do relative to what's getting rotated: the arm in the shoulder joint.

An image may help. Take a look at the relative positions of the scapulae in the picture to the right here (in red the supraspinatus is highlighted, but that ain't important right now). If we look at the left arm, we'll note that the glenoid fossa - where the humerous fits into the shoulder) is in neutral. With the right arm elevated, like in an overhead press, we see that the whole scapula is titled away from the spine and that the glenoid fossa - where the arm connects - is pointed more UP.

Bottom wonderful line is that, by rigging up the scapula so that it's got all these guy wired muscles holding the scapulae as a kind of floating anchor point, we get far greater range of motion with our arms than if we had a more or less fixed ball and socket joint.

That is, if the scapula, which is largely designed to act as an attachment for the upper limbs, were fixed to the spine as a bone such that the socket for the humerus was fixed ( unable to rotate up, down, back and forward), the arm would have far more restricted motion. We'd be unable to press up, cross our arms, do push ups, waltz. Awful to contemplate.

So let's not. Let's sum up where we're at today.

Summing up part 1
In this first article on the shoulder girdle & scapula, we've had a quick look at the muscles that support the movement of the scapula and concurrently the resulting rotation of the glenohumeral joint that allows for the all important rich range of motion of the arm at the shoulder.

Next, we'll look at the muscles acting on the glenohumeral joint, like the giant lats, pec major, teres major, and including those pixie trouble makers, the rotator cuff muscles. Guarenteed once we go over how they work and where they are in the scapula, remembering the names of the four will be simple.

But heck, isn't that scapula an amazing bone or what, eh? 

Some great books to help get into structural kinesiology are

Manual of Structural KinesiologyManual of Structural Kinesiology
Anatomy of Movement (Revised Edition)really really good source for getting at the complete details on joints, muscles, actions, planes of motion, nerves. THere are great exercises and quizes with the book as well for self-testing

Anatomy of Movement (Revised Edition)
A very personable look at anatomy in the context of not only athletic but every day movements.

Trail Guide to the Body BookAnd for getting into the actual feel of where these muscles are and how they operate live, there is the very popular Trail Guide to the Body Book

And heck if you've ever tried to figure out whether that groin pull is an adductor magus or gracialis, this is the one for you.

More books that include seeing the real tissue (making the case for illustrations) next time.

Related posts:

1 comment:

Lynn Weddle said...

I spent 4 months in 2009 with shoulder impingement and while it's better, stronger, it's a work in progress. Understanding how it works is so important... then I want to understand how most people can ultimately not take care of the shoulder and get hurt. I thought I was in great shape and strong in my upper body but come to find out I just was not smart. Anyway, thanks for the 2 articles!


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