What structures make up the shoulder girdle?
The arrangement of bones and muscles in the shoulder girdle allows us to do a wide variety of movement, possibly a greater variety of movement than anywhere else in the body. Let’s explore the bones and muscles that make up the shoulder girdle, which is also called the shoulder complex.
Bones in the shoulder girdle
The shoulder complex includes several different bones. Some references refer to the shoulder complex as composed of the scapula, clavicle, and sternum. Others include the humerus, scapula, and the clavicle. It makes sense to me to include the humerus as part of the shoulder girdle, as the shoulder joint itself (the glenohumeral joint) is where the humerus and scapula meet. The more important point to take away is that the movement and position of the scapula will directly impact the movements available at the glenohumeral joint (shoulder joint) and vice versa. They are intimately linked.
Specifically, the shoulder joint itself, called the glenohumeral joint, refers to the joint where the head of the humerus (the upper arm bone) meets the glenoid fossa of the scapula. However, the position and movements of the scapula affect the movements of the humerus, so all of these structures are often referred to collectively as the shoulder girdle or shoulder complex.
We have potentially four bones that could be considered part of the shoulder girdle or shoulder complex:
- The humerus = the upper arm bone
- The scapula = the shoulder blade
- The clavicle = the collarbone
- The sternum = the breastbone
Joints in the shoulder girdle
We have three or four (depending on how you describe them) joints where movement can happen in the shoulder girdle:
- The glenohumeral joint = shoulder joint where the humerus meets the glenoid fossa of the scapula
- The acromioclavicular joint = where the acromion of the scapula meets the clavicle
- The sternoclavicular joint = where the clavicle meets the sternum
- The scapulothoracic joint = some references also refer to the scapulothoracic joint. Although not a true joint, this is where the scapula meets the ribcage.
The sternoclavicular joint is particularly important as this is where the shoulder girdle, which is part of the appendicular skeleton, attaches to the axial skeleton.
Movements of the shoulder girdle
The individual movements of each joint of the shoulder complex combine together to create the variety of movement that is possible at the shoulder girdle as a whole. Movements of the humerus at the glenohumeral joint include: flexion, extension, abduction, adduction, lateral rotation, and medial rotation. We discussed the movements of the scapula already in the article on exploring the scapula. Movements of the scapula include: elevation, depression, protraction, retraction, upward rotation, downward rotation, and tilting. The movements of the clavicle are similar to the scapula. The movements of the clavicle include: elevation, depression, protraction, retraction, upward rotation, and downward rotation.
Some researchers have suggested that, evolutionarily, our upper extremities evolved to place the hands in space, while the lower extremities evolved to take us places, i.e. to walk and run. With this idea in mind then, we can see why such a range of mobility in all planes of space would be necessary for the shoulder girdle, while less mobility, but more strength and power in the hip joints, would support walking and running.
The shoulder girdle uses multi-joint, three-dimensional movements to position the hand. The fine movements we take for granted everyday, from typing and writing, to using our cell phone or playing the piano, all originate with movements of the shoulder girdle which position the forearm, and ultimately the hand and wrist. When we raise our arm overhead, this motion does not come from the glenohumeral joint alone. The scapula and clavicle also move to allow the arm to be elevated (Ludewig et al., 2009). In fact, the full kinetic chain from the axial skeleton to the hand is really: thorax/ribcage → scapula → upper arm → lower arm → hand (van Andel et al., 2008).
You can see why such variety of movement is possible in the shoulder girdle when we combine all these uniquely moving pieces. If you want to read more about the shoulder girdle check out this related article: Exploring The Scapula.
References
Ludewig, P.M., V. Phadke, J.P. Braman, D.R. Hassett, C.J. Cieminski, and R.F. LaPrade. 2009. Motion of the shoulder complex during multiplanar humeral elevation. The Journal Of Bone And Joint Surgery. 91-A(2):378-389.
van Andel, C.J., N. Wolterbeek, C.A.M. Doorenbosch, D. (H.E.J.) Veeger, and J. Harlaar. 2008. Complete 3D kinematics of upper extremity functional tasks. Gait And Posture. 27:120-127.
