Shoulders play such a crucial role in our daily lives and we barely notice them until they start to give us trouble. We use them on a daily basis as they anchor our arms and help us support our bodies. And sometimes… They are that little edge on our body that someone can lean on in tough times. Yes, shoulders are great, but sometimes they give us a little trouble from time to time. Let’s get to know these a little more, shall we?
Shoulders are the most mobile joint in the body. This is because of its range in multiple planes of movement. When we talk about the mobility of a joint, we have to consider another aspect too, stability. In the world of joints (and life in general if you actually think about it) there is this precious tension between mobility and stability. The more mobility you have in a certain joint, the less stability, and the more stability you have the less mobility you have. It is for this reason that the shoulder is also one of the most unstable joints in the body.
Generally, when we talk about the shoulder the focus on one main joint, which is the glenohumeral joint. This is a ball and socket joint between humerus (arm bone) and scapula (shoulder blade). In reality, the shoulder is actually made up of 4 joints: the sternoclavicular (joint between your breastbone and collarbone), acromioclavicular (scapula and collarbone), scapulothoracic (shoulder blade and chest wall) and glenohumeral joint as we mentioned above. All these joints contribute to the movement we have in the shoulder and is also why we can never look at issues at the shoulder in isolation.
The high mobility of the shoulder joint is mainly because of the glenohumeral joint, where the head of the humerus fits into the socket of the scapula (the glenoid fossa). Now, the glenoid fossa is smaller than the head of the humerus, and is fairly shallow, which allows for the amount of mobility in the joint. This is also the reason why this joint is highly unstable. (Stability can be described as how hard it is to move the joint from its centralized and ideal position and how easily the joint maintains this position, especially during movement.)
Now when we look at the shoulder, a lot of its stability comes from other structures.
We divide the stability into two factors, namely, passive stability and active stability.
Passive stability is attained through structures that cannot be actively controlled, ie the shape of the joint, joint cartilage, joint capsule and membranes as well as ligaments, bursae and even the fluid inside the joint.
Active stability is one we can, to a certain extent control actively, this is attained through muscle action on the shoulder joint. The primary stability is from the rotator cuff in the glenohumeral joint, however the full shoulder girdle gets its stability through the action of 17 muscles in total.
Another component often overlooked when talking about shoulders is that of the neurovascular bundles, this refers to the nerve and blood supply of an area. The nerves guide the activation of the muscles which then plays a role in when different muscles fire and how, as well as in different sensations such as touch, pressure and pain. The blood supply is responsible to carry nutrients and oxygenated blood to the area and removing waste substances. Our neurovascular bundle plays an important role in our process of maintenance, experience of injury, healing and recovery.
This article is a brief overview on what is going on in a normal shoulder, it hasn’t factored in any dysfunction yet. We can see why the complexity of the shoulder means it is so important to get a correct assessment done to figure out what is actually causing a shoulder injury.