The complex construction of the shoulder complex provides a unique mobility that surpasses the mobility of any other joint in the body. While it incorporates a ball-and-socket joint, similar to the hip, the shoulder joint has a more limited bony interface which provides much greater mobility than the hip joint, another ball-and-socket joint. The shoulder must be mobile enough for the variety of movements required of the arms and hands, but also stable enough to allow for actions such as lifting, pushing and pulling.
The shoulder girdle acts as a movable but steady support for the motions of the humerus. The shoulder girdle has 2 bones (clavicle and scapula) that move together as a unit. To maintain its integrity, the shoulder girdle depends more on muscles than on joint structures. Because the upper limb is not usually involved in weight bearing, its stability has been sacrificed in exchange for greater mobility. Such a construction provides dynamic stability to the complex, i.e. greater mobility and stability at the same time through muscular contractions.
The arm is connected to the trunk through the shoulder mechanism, a constellation of four bones (humerus, scapula, clavicle, and thorax). There are 4 different articulations that make up the shoulder girdle complex:
- Glenohumeral – the synovial ball and socket joint articulation between the glenoid fossa of the scapula and the head of the humerus. This joint is what most people associate with the shoulder joint. The surface arae of the humeral head is much larger than the surface of the glenoid fossa, and only a part of the humeral head touches the glenoid. Thus, the joint is inherently unstable. The glenohumeral joint has 3 rotational degrees of freedom (DOF).
- Acromioclavicular (AC) – aka claviculoscapular joint, at the top of the shoulder between the acromion of the scapula (highest point of the shoulder) and the lateral clavicle. In the transverse plane, the angle between the scapular spine and the clavicle is approximately 50 degrees. In the joint, the scapula moves on the clavicle. The joint has three rotational DOF.
- Sternoclavicular – compound joint in which the clavicle articulates with the manubrium of the sternum and the cartilage of the first rib. The joint cavity is divided into two compartments by the articular disk. The sternoclavicular joint is the only joint that connects the shoulder complex to the axial skeleton.
- Scapulothoracic – formed by the female surface of the scapula and the male surface of the thorax. This is a bone-muscle-bone articulation that is not synovial and, in the true sense, is not a joint. However, considering it a joint is of some value when describing motion of the scapula over the thorax. The mating surfaces are separated by the subscapularis and serratus anterior muscles, which glide over each other during motion. The scapulothoracic articulation allows translation and rotatory movement of the scapula with respect to the rib cage.
The scapula lies obliquely over the ribs between the frontal and coronal planes. The angle between the frontal plane and the plane of the scapula, called the protraction or winging angle, is typically between 30°-45° with an average of 35°. The scapula is also anteriorly tilted 10° and upwardly rotated 5°. The plane of the glenoid is approximately at right angles to the plane of the scapula. The scapula has no bony and ligamentous connections with the chest.
The following video provides a very good review of the shoulder girdle.
To really understand how the scapula moves, we need to understand what muscle groups are involved. Scapular rotations are so complex because there are 17 different muscular attachments, which can be classified by three different groups of muscles:
- Intrinsic Muscles – attach scapula to humerus;
- Rotator Cuff (Subscapularis, Supraspinatus, Teres Minor, Infraspinatus) – active stabilization of shoulder joint by preventing downward displacement of humerus
- Teres Major
- Extrinsic Muscles – attach/suspend scapula to neck and trunk; stabilize and/or actively moves scapula in a variety of movements
- Muscles of the Shoulder and Arm
- Triceps Brachii (long head)
- Biceps Brachii (long and short head)
The scapula exhibits an extremely wide range of motion due to a very limited bony interface between the humerus and glenoid fossa of the scapula. The video below provides a very good animation of scapular rotations including:
- Elevation (Upper Traps, Levator Scapulae, Rhomboids) / Depression (Lats, Pecs)
- Upward Rotation (Serratus Anterior, Upper and Lower Traps) / Downward Rotation (Lats, Pecs, Levator Scapulae)
- Protraction (Serratus Anterior, Pecs) / Retraction (Traps, Rhomboids)
Special Anatomical Consideration – Lats
One of the most misunderstood muscles that affects scapular rotations during overhead throwing is the lats. In fact many anatomy books even describe that the lats often overlay the lower portions of the scapula with no direct attachment. But the lats do attach to the inferior angle of the scapula – at least in a certain percentage of the population. This article by Dan Ogburn describes how Pouliart et al investigated the anatomy of latissimus dorsi in 100 cadavers and found that in many specimens there was a connection to the scapula as the muscle ran its course to ultimately attach on the humerus. They described three types of interactions between lats and the inferior angle of the scapula:
- Type 1, Direct Attachment: muscular slips are anchored directly onto the inferior angle of the scapula.
- Type 2a, Indirect Attachment: few muscle fibres with a soft, fibrous link between the two (fascia).
- Type 2b, No Attachment: a bursa and no connective tissue between the two.
Ogburn adds that the majority of people (79%) had either a Type 1 (direct, 43%) or Type 2a (indirect, 36%) attachment, while the remaining 21% had no attachment at all (Type 2b). So it seems that in the majority of the population, there is some form of link between the latissimus dorsi and the scapula, even if some of us have no direct interaction.
But the effect of the PAS lats on scapular rotation during the dynamic pitching motion is very real and can be very problematic. I consider Eric Cressey to be the foremost expert in overhead sports and specifically for baseball pitching. Eric has done a number of articles and videos on the effect of short/stiff lats on depressed shoulders/scapula on PAS. In baseball throwing, lat recruitment has been shown to be higher during acceleration in professional pitchers than amateurs. Thus the lats are a very important muscle group in generating increased pitching velocity.
I look in more depth at Eric’s work with the effect of lats on PAS scapular/shoulder depression in the PAS scapular rotations section, but it is important to first look at Eric’s discussion of the function of the lats in general athletic movements in this section. Eric has an excellent article on the function of the lats and their importance in athletic function as they are the biggest muscle in force transfer between the lower and upper body. The reason why the lats are so important in force transfer between the lower (pelvic) and upper (shoulders) body is the vast cross-sectional area of the muscle group. The great thing about the lats is they are very easy to train and have serious growth potential due to their size.
There are 5 different points of attachment for the lats:
- Pelvic: posterior aspect of the iliac crest
- Vertebral: T6 all the way down to the sacrum
- Ribs: last 3-4 ribs
- Scapula: inferior scapular border (as discussed above)
- Humeral: intertubercular groove of the humerus.
One important thing to consider with the lats as discussed by Eric Cressey is the orientation of the muscle fibers. The lats have a diagonal fiber orientation which allows for more complex movement and stability demands compared to muscle groups with purely horizontal (pecs) or longitudinal (biceps, hamstrings) orientations. This provides for potentially more powerful force transfer between the upper and lower body through the lats. The lats attach with several other muscles into a large bunch of connective tissue called the thoracolumbar fascia which has pelvic, sacral, and vertebral attachment points. Thus the lats work with other core stabilizers to transfer force between the upper and lower body. The lats also function as powerful multi-directional stabilizers of the lumbar spine due to the thoracolumbar attachments as well.
From a functional biomechanics standpoint, the lats are responsible for humeral extension, humeral adduction, humeral internal rotation, and humeral horizontal abduction. However, proper pitching mechanics requires the humerus to flex, abduct, and externally rotate. Thus, the lats are a direct antagonist to healthy overhead biomechanics.
Eric Cressey has said that people tend to overlook the effect of the lats on the position of the scapula. The lats not only influence the scapula indirectly via its functional biomechanics effects on humeral motion as described above, they also directly (albeit somewhat weakly) are involved in some important scapular motions via the subtle attachments on the scapula as it travels to the humerus.
As one of the scapular depressors, the lats work to counteract the upper traps and levator scapulae, both scapular elevators. The upper traps and levator scapulae can cause big problems with respect to forward head posture and pain along the medial scapular border. By making the lats stronger, one can help to re-position the scapula to offset these problems. But the opposite is also true; if the scapular elevators are weak then the head will fall behind the rapidly unwinding torso.
Along the same lines, the lats are weak scapular retractors, meaning that they assist the entire trapezius complex and rhomboids in pulling the shoulder blades together. The lats also help to pull the scapula down as described above. And for the majority of athletes that train a lot with bench press, this retraction helps to counteract the shortness these athletes have in their pecs. So the lats really help the general population that need to get the scaps “back and down” to improve posture.
Lastly, the lats help the levator scapulae, rhomboids, and pectoralis minor out in downward rotation of the scapula. If the lats are short and stiff, they will contribute to scapular downward rotation syndrome. This is a common problem in athletes that have tight lat muscles and can’t upwardly rotate the scapula properly.