The shoulder joint proper, or glenohumeral joint, is unique in its ability to move in multiple planes in large arcs (the hip, ankle, wrist and jaw also move in multiple planes, but less so). It consists of the head of the humerus (upper arm bone) and the scapula (shoulder blade). The humeral head inserts into a shallow indentation of the scapula called the glenoid fossa, and is positioned there by the capsular ligament (labrum) and rotator cuff: the tendons and muscles surrounding the glenohumeral joint that are responsible for moving it. These muscles are the:
- Teres Minor
Each of these muscles start (originate) somewhere on the scapula, and connect (insert) around the humeral head. The orchestration of muscle firing is what gives the glenohumeral joint its impressive range of motion.
But because of the wide range of motion, there is greater risk of stress to this joint. For example, a tennis serve, golf swing and baseball pitch are (typically) repetitive movements that can cause accelerated wear and tear to the rotator cuff. The tendons, capsular ligaments and cartilage lining the joint surfaces can tear, resulting in pain, inflammation and limited mobility.
A ligament can thicken and get pinched by the bony structure just above the humeral head called the acromion as the arm is abducted (raised).
However, there are many cases of people who have no history of such trauma to their shoulder, yet still develop shoulder pain. What can be the cause?
How Lack of Motion Can Lead to Pain
While shoulder joint-dependent sports like tennis and golf can lead to joint damage, the exact opposite is true as well: shoulder relative immobility can lead to joint pain. Here’s how.
The shoulder joint is a synovial joint. Synovial joints are enclosed by a capsule lined with synovium, a thin layer of tissue containing special cells that secrete synovial fluid. Synovial fluid, which is viscous in nature (like egg yolk consistency) is comprised mainly of hyaluronic acid and lubricin, two components whose functions are to absorb shock and reduce friction.
Not only is the synovial fluid responsible for smooth joint movement, it also provides nutrients and oxygen for chondrocytes and fibroblasts— the specialized cells that make the cartilage material (matrix)– and serves as the means to remove waste products from the joint capsule environment. As you can imagine, synovial fluid is critical to the proper functioning of the joint.
As the humeral head glides over the glenoid fossa of the scapula, the pressure squeezes out old synovial fluid from the cartilage lining and the cartilage then absorbs fresh synovial fluid. And here is where the problem lies.
If the shoulder joint is not moved enough, the cartilage lining the joint will deteriorate faster. The synovial fluid becomes more stagnant, less of it is produced, and the chondrocytes that make the cartilage start to die leading to thinner, weaker cartilage that is more susceptible to tearing and pitting.
Think about it– if you don’t play sports or exercise, there is little need to raise your arm above shoulder level throughout your day. Most of the things in life we handle are below shoulder level. Raising your arm from your side to pointing straight up represents a full arc, where much of the surface of the humeral head glides over the glenoid fossa of the scapula, “bathing” it in fresh synovial fluid. This movement is what is missing for those who have “sedentary shoulders,” which is the case when you sit at a desk and type away at the computer— there is minimal to no shoulder joint movement during typing.
People in this situation are more prone to “frozen shoulder,” or adhesive capsulitis, and then degenerative joint disease and eventually chronic shoulder pain and limited shoulder mobility.
The solution is, of course, to move those shoulders frequently! Here is a video of exercises that will do the job quite nicely: