Many people think that the shoulder girdle is represented by a single joint. But in reality, the upper limb girdle consists of four joints that are combined into a complex multi-articular structure. What does it consist of?
- Glenohumeral joint (shoulder joint). This is the connection between the humerus and scapula and is considered the "true" anatomical joint. It is the most important joint of the shoulder complex.
- Acromioclavicular joint. This is the anatomical joint that connects the clavicle and the acromion.
- Sternoclavicular joint. This is also a true joint connecting the clavicle and the sternum.
- Scapulothoracic "joint." This is a "false joint" that does not contain cartilage tissue but represents a mobile connection between the scapula and the ribcage. Its function depends on the interaction between the sternoclavicular joint and the acromioclavicular joint.
How does it work?
Let us consider shoulder abduction as an example — this is the movement of the upper limb away from the trunk, occurring in the frontal plane around the sagittal axis. Movement from 0 to 60 degrees occurs mainly due to movement at the glenohumeral joint; from 60 degrees upward, the scapulothoracic "joint" actively engages, followed by the acromioclavicular and sternoclavicular joints. It is important to understand here that from the first degrees of movement, the scapula will be involved to some degree, which means movement at the scapulothoracic "joint" is an important element in the proper functioning of the shoulder complex. Thanks to scapular movement, a large range of motion is possible at the glenohumeral joint.
It turns out that all joints start simultaneously, but the percentage ratio will differ depending on the phase of movement.
The joints of the shoulder complex are interconnected and function in synergy. Any movement in the shoulder girdle, from scratching to complex coordination movements (for example, the jerk in weightlifting), involves all joints to varying degrees depending on the type of movement performed.