I want to understand my shoulder better.
The shoulder is made up of 4 joints consisting of the interface of 4 bones and the ribcage. The glenohumeral joint is the actually ball in socket “shoulder joint” which is the interface of the humerus (arm bone) and the scapula (shoulder blade). The scapula has a very shallow bony interface and the glenoid cavity (see images below) for a relatively large ball (humeral head) to glide around in. This accounts for the large range of motion that the shoulder has at the cost of a decrease in stability as compared with our other ball in socket joint – the hip.
A healthy shoulder should be capable of 180 degrees of flexion and abduction, 60 degrees of extension, 90+ degrees of external rotation and 70-80 degrees of internal rotation. Oftentimes, when an individual is lacking in one range of motion they make up for it or compensate with another. The classic sports specific compensation is with baseball pitchers who have an extreme amount of external rotation to generate torque in their throwing arm but lack normal range of internal rotation. However, their range of motion combining internal and external is more than likely still around 160 degrees.
The streno-clavicular joint is the true attachment of the joint complex to the rest of the body. This is a very stable joint (as it needs to be) and is rarely the site of dislocation. The acromio-clavicular (AC) joint is a weak gliding joint, which is the combination of the acromion processes of the sapula and the distal end of the clavical. The glenohumeral joint is interesting in its rolls and glides, which are kinesiological terms for the way the proximal portion of a joint moves relative to the physiological movement of the joint. For example. As the humerus moves into flexion the ball (humeral head) in the socket (glenoid cavity) has a component posterior glide. This is an important concept as restoring and maintaining neutral joint alignment is pivotal for maintaining a healthy shoulder.
Another interesting concept is the idea of scapulo-humeral rhythm. There is 2x more motion at the glenohumeral joint than the scapulothoracic (estimation, not a constant ratio in vivo). So if we look at 120 degrees of physiological flexion or abduction, it is the combination of 40 degrees of scapula upward rotation (scapulothoracic joint) and 80 degrees of glenohumeral flexion or abduction. Broken down further, the 40 degrees of scapula upward rotation is made up of 20 degrees of strenoclavicular elevation and 20 degrees of acromioclavicular elevation. It should not be too complicated to see that mobility or loss of mobility at any one of these joints will have a drastic impact on obtaining full range of motion at the shoulder joint.