My last blog left us with a beautiful photograph showing us an un-embalmed cadaver dissection of the serratus anterior muscle, from the Bertelli/Ghizoni 2005 paper.
As promised, further *morphological information about this muscle is provided in today's blog.
*The word part ‘morph’- means "form" and ‘ology’ means "the study of” – vocabulary.com
If you are interested in reading further, the links for these three papers are provided above. Also, I share more information on this muscle and how it might work, in my Scapulothoracic Assessment series (webinar #2), available through embodiaapp.com
Of the three papers being presented today, the study by Webb et al. provides us with the most information because they examine the whole muscle - the upper, middle, and lower divisions, and they provide direct measurements of fascicle* orientation, length, thickness, tendon length and physiological cross-sectional area (PCSA). Their very close inspection of the upper and middle parts of this muscle has led to a better understanding of its attachments.
*fascicle - a small bundle of skeletal muscle fibres surrounded by connective tissue.
The serratus anterior muscle attachments to the ribs are understood to be always on the superior aspect, but Webb et al. describe two fascicles originating from the second rib: one attaching to the superior aspect and one to the inferior aspect. The rib two superior fascicle (R2s) joins the rib one fascicle (R1) to form the upper division and the inferior fascicle (R2i) combines with the rib three fascicle to form the middle division.
The middle division follows a horizontal, divergent path, then attaches to nearly the entire medial scapular border. It is the thinnest division at its midpoint, but with the largest overall PCSA, its overall fascicle volume is consistent with that of the upper and lower divisions.
Webb et al. 2016 R2s - rib 2 superior fascicle, R1- rib 1 fascicle, R2I - rib 2 inferior fascicle
The upper division, also shown in this Webb study photo, above, is perhaps best represented by the photo below, from the Smith et al. study.
Notice the thick, cylindrical shape of the upper serratus anterior. Its mean length is 6.9 cm, and its mean girth is 6.1 cm. This is approximately as thick as one of your fingers!
While it appears small overall, this upper-division is thick and powerful, like a piston.
Webb et al. determined that while the shape and length of fascicles varied between divisions, the number of fascicles PER RIB was consistent. 7 for rib one, 8 for all other ribs except rib 9 which had 4 fascicles. This is a symmetrical, well-balanced, muscle.
Smith et al. 2003
Theoretically, the forces generated in the upper division can be transferred through the middle division and balanced by the forces generated in the lower division - and vice versa.
These last two photos from the Webb et al. study, show attachment points on ribs #1-8, (L) and the entire muscle in situ,(R).
Attachment points on ribs #1-8 (muscle removed) Entire muscle in situ
(both photos: Webb et al 2016)
Notice the large lower division (R4 – 8), starting at the lower scapular angle, with fascicles travelling anterior and inferior (forward and downward) to their individual ribs, at an angle varying from 28 to 78 degrees.
Connections above and below
Webb et al. reported that in 75% of muscles examined, the rib 2 superior fascicle attachment extended to the fascia of the first intercostal muscle. This connection is shown in the bottom left-hand corner of the first Webb et al. photo, above, and is labelled - 1st IC muscle. This suggests a significant physical connection that will contribute to lifting the first and second ribs.
Webb et al. also reported that the lower four fascicles of the serratus anterior interdigitate with the fascicles of the external oblique muscle, suggesting both a physical and functional ‘link’ that is essential if we are to better understand the human KINETIC CHAIN.
While I find this muscle fascinating to study, I am sure that most readers are wondering at this point… “Why are these morphological details so important?”
This muscle is poorly understood. So far, Electromyographical (EMG) studies have been limited to the exposed, accessible parts of the serratus anterior. As a result, only one segment of this muscle, usually at its eighth rib attachment, is sampled when serratus anterior muscle activity is examined. Even from this relatively ‘exposed’ location, the number of fibres sampled is very small and the standard surface electrode location can sometimes be unreliable, especially with specific movements and arm positions (Richardson et al. 2020 referencing Hackett et al. 2014).
Trapezius, also a large, important scapulothoracic muscle, is superficial and is therefore much easier to examine. Surface electrode placement is easy. As a result, EMG studies of “scapulothoracic muscles” routinely measure activity at three different trapezius locations, (upper, middle and lower divisions), and only one, sometimes unreliable serratus anterior location. The data we have, so far, about scapulothoracic muscle activity, is promising. We are starting to understand how this kinetic chain works, but we do not, yet, have the full picture.
If you take a final look at the photo from Bertelli et al. (shown at the beginning of today’s blog),
You will see that there are distinct, separate, nerve branches supplying different parts of the serratus anterior muscle. The upper division has its own branch, and the lower division has separate branches that supply muscle fascicles at each rib level. Is this important? We do not know. Do each of the divisions have a separate function, or does the muscle just contract as a single entity? We do not know.
This is a complex, three-dimensional muscle that plays a significant role in linking the scapula to the thorax (chest wall). We measure only one small segment, and our ability to do so is inconsistent and sometimes unreliable. There is still much that we need to learn about how this muscle works. Taking a closer look at the structure, shape and form (morphology) of serratus anterior is an important first step. It can lead to better research questions and more effective use of diagnostic and research tools. At this time, EMG is the only tool used regularly for research and diagnosis, but perhaps better options will become available once we have a better understanding of how this muscle works and where we need to be looking.
Strength training using the kinetic chain is a topic that is now being investigated and studied by researchers, especially by those who have a special interest in sports medicine.
Serratus anterior plays a part in this very important linkage system. Our next blog will investigate what the current research can tell us about this role, and what the current research is still struggling with.
My view from the “inside” of the body is unique. I have learned that the kinetic chain is not entirely a “proximal to distal” mechanism that exists solely to serve the glenohumeral joint. It works both ways and can provide support and strength to many areas of the body. It is not ONLY about the glenohumeral joint. Reducing impingement and supporting the optimal position for the head of the humerus is one benefit of an effective scapulothoracic ‘link’ in the kinetic chain, but it is not the only one. This is a much bigger story. More details to come…