Oversimplified at best and overlooked at worst, the shoulder blade plays a crucial role in shoulder mobility. Understanding how the the ribs and shoulder blades lay the foundation for healthy shoulders can save you an awful lot of time and pain.
Defining the Shoulder
Definitions are rarely exciting, so let's get this one out of the way: the shoulder joint is more technically known as the glenohumeral joint. Described as a ball and socket joint, the ball is the head of the humerus (upper arm), while the socket is the glenoid fossa, a shallow concavity on the shoulder blade. And while the joint itself is the single most mobile joint of the body, its mobility is increased by good scapular movement. This is particularly important when you consider the structure of the joint—the shallow socket allows for greater freedom of movement at the expense of stability, making alignment and position an important consideration in upper body strength training.
Drawing an Analogy
I'm anything but an expert on maritime fighting techniques, but I think I've got the gist of things:
steer the boat,
aim the cannon,
fire.
It's a strange analogy, but I think it works: imagine your arm as the cannon (you know you want to), your shoulder blade as the boat, and your ribcage as the surface of the water. You can aim the cannon on its own, but you increase its range of motion (and avoid damaging the mount) by turning the boat, which, in turn, depends on how smooth the seas are. Marrying all three elements—in our case the ribs, the scapula, and the humerus—provides for better mobility and ensures that when that cannon goes off it doesn't go flying off of its mount. The joint's range of motion hasn't changed, but the joint itself is able to move to better accommodate the necessary end position.
The Motion of the Ocean
A deeper understanding of how these three elements interact requires an understanding of exactly how they move, and a brief look at some of the interplay between these movements.
Before we get into the specifics, a note on planes of motion and anatomical positions: these are imaginary constructs and grossly oversimplify human movement. It's harder than you might think to find a compound movement that exists within one plane of motion. We don't need to get too deep into this, but if you're bothered by seeing movement described as a blend of sagittal, frontal, and transverse planes motion just let it go—reality is a whole lot messier than the pages of a textbook.
With that out of the way, let's look at the movements each structure is capable of.
The Ribcage (Multiple Joints)
While the movements of the ribcage are certainly subtler than those of the shoulder, good rib movement and position underpins nearly every movement of the upper body. Movement of the upper ribs is often described as a "pump handle" action as they move up and forward, while the the lower ribs move up and out in more of a "bucket handle" fashion. As illustrated by the animation, these movements are truly triplanar, made up of a combination of flexion/extension, abduction/adduction, and even internal/external rotation.
The movement of the upper ribcage influences the position of the scapulae and how freely they can slide across its surface. The anterior aspect of the scapulae is concave, when the ribs don't present a complementary convex surface to the inside of the scapulae their movement is limited, and, as a result, upper limb movement is either restricted, or has the potential to place undue stress on the tissues around the shoulder joint.
So how do we ensure that the ribs are in position to allow for full shoulder range of motion? By learning to use breath to shape our ribs. An in-depth examination of everything that goes into breathing is well beyond the scope of what we're looking at here (it could and has filled hundreds of pages). Instead, let's focus on a few key concepts:
Belly breathing is bogus; your lungs are in your chest, not your belly.
The ribs can and should expand in all directions as you inhale.
When it comes to scapular movement it's particularly important to breathe into your upper back to help keep the upper ribs from sitting too flatly.
Your abs—particularly your internal obliques—need to provide just enough resistance to assist diaphragmatic action by holding the lower ribs in place, ensuring proper positioning of the thorax for efficient breathing.
The Scapulae (Scapulothoracic Joint)
The scapulothoracic joint is nothing if not atypical. Less of a true joint and more of a bony articulation, it has no supporting ligaments and is instead held together by surrounding musculature. The same muscles responsible for the moving the shoulder blade also keep it in position. My belief is that this increased reliance on musculature for bony position makes upper extremity dysfunction particularly common, one more reason it's worth giving a muscle like the serratus anterior some extra love.
There are eight distinct movements of the scapula, often occurring in concert with each other as well as with movement at the glenohumeral joint. The movements of the scapula are:
upward and downward rotation,
elevation and depression,
protraction and retraction,
anterior and posterior tilt.
We'll get deeper into how these movements work to assist in upper arm mobility in a bit, but for now let's look at how the upper arm itself moves.
The Humerus (Glenohumeral Joint)
In a nice bit of symmetry the shoulder joint is also capable of eight distinct movement patterns. While these are likely more familiar, they're worth reviewing before moving on:
flexion and extension,
internal and external rotation,
abduction and adduction,
horizontal flexion and horizontal extension.
Rhythm and Base
I started with the assertion that an understanding of the interplay between the ribs, the shoulder blades, and the arms could help you move better and with less risk of injury, and introduced the analogy of the sea, a ship, and a cannon as stand-ins for our anatomical structures. Let's revisit the analogy and see if I can make good on my initial promise.
The effectiveness of a cannon is limited by where you can point it. Turning the ship expands the range of motion, which is aided by smooth seas. Conversely, turning the ship the wrong way effectively reduces the cannon's range of motion, while rough seas can make maneuvering our ship difficult. In anatomical terms we need an smooth base (the posterior surface of the ribcage) to maneuver our scapulae on. In turn, our scapulae need to move in appropriate rhythm with our upper arms, as moving the wrong way can actually make things more difficult.
In most movements, pairing the following joint actions allows for greater range of motion and reduced stress on the glenohumeral joint:
shoulder abduction and scapular upward rotation,
shoulder adduction and scapular downward rotation,
shoulder flexion and protraction/upward rotation,
shoulder extension and retraction/downward rotation,
shoulder horizontal flexion and protraction,
shoulder extension and retraction.
(*While the scapulae are capable of tilting, this is ideally a particularly subtle motion and one that allows the scapula to sit flush on the ribcage, hence the exclusion of this particular movement.)
Other Considerations
"All generalizations are false, including this one." Mark Twain
While I'm clearly a proponent of allowing the scapulae to move, there are clearly exceptions to every rule. Sometimes keeping your shoulder blades down and back in a "packed" position is your best bet. And honestly, it's less important to me to establish a set of "rules" to be followed than it is to introduce some ideas to be considered; "packing your scap" may work well for a wide grip military press, but not as well for a handstand pushup, and while it's probably a good idea for a farmer's walk or a yoke carry, it may or may not work work for your deadlift. There's plenty of fitness dogma out there without me adding to it.