The serratus anterior has complex and variable anatomy. It is a broad, flat muscle with multiple bellies. It usually has eight sections, connecting the scapula’s vertebral border to each of the first eight ribs. Therefore, this structure makes it an extrinsic chest muscle. It is on the lateral trunk, under the scapula, extending like a fan through the axilla onto the anterior thorax.
It has three sections named for their position: superior, intermediate, and inferior.
- superior section – anterior aspect of the first and, usually second rib
- intermediate section – anterior aspect of ribs 3-5
- inferior section – anterior aspect of ribs 6 – 8.
- superior section – superior angle of the scapula
- intermediate section – vertebral border of the scapula
- inferior section – inferior angle of the scapula
All heads protract the scapula but have an additional function.
- superior – protraction and downward rotation of the scapua
- intermediate – protraction and elevation of the scapula
- inferior – protraction, depression and upward rotation of the scapula
- Long thoracic nerve, C5-7
The upper section has transverse bellies that extend between the upper scapula and ribs. This section attaches anteriorly to the first and second ribs and the fascia of the thorax. However, there is some variance in the rib attachments. In fact, the superior section does not always extend onto the second rib.
The middle section originates at the vertebral border of the scapula. The first vertebral belly attaches to the upper vertebral border and extends to the nearby 2nd rib. As the origin continues down the vertebral border, fibers extend upward at about 45-degrees. Consequently, these fibers get progressively longer as they extend toward a more anterior aspect of the 2nd rib. This structure forms a flat, triangular lamina. Next, the origin of the middle belly extends further down the vertebral border. Those fibers extend up to attach to the 3rd rib. This insertion is shorter than the insertion of the belly attaching to the 2nd rib. Finally, the origin of the lower belly originates near the inferior angle of the scapula. It inserts along the 4th rib in an attachment that is shorter than the belly above.
The middle bellies attach along the medial border and elevate the scapula. At the same time, the scapula moves anteriorly so that the shoulders shrug. This posture appears as elevated shoulders with a caved chest.
The lower section has finger-like projections that originate from the lower angle of the scapula. These bellies extend anteriorly and inferiorly. They connect the lower angle of the scapula to the 6th, 7th, and 8th ribs. They are visible as the chevrons on the lower lateral ribs of the anterior thorax. There may be slips that attach as low as the 12th rib. One study revealed that they vary from one side to the other in the same cadaver 30% of the time.
The lower section draws the lower angle toward the lower ribs. This action moves the scapula anterior and inferior. It also assists in the upward rotation of the scapula.
Breathing and Posture
Serratus anterior lifts the rib cage in combination with muscles like the sternocleidomastoid and scalenes. When the diaphragm is weak or inhibited, the serratus anterior accommodates breathing. Practically put, these muscles help you breathe when you are hunched forward at your desk or slumped on the couch.
When the extrinsic back muscles stabilize the scapulae, the serratus anterior expands the chest cavity. It, with pectoralis minor, is positioned to both elevate and widen the thoracic cavity. As a result, it dramatically increases lung capacity during labored breathing. With the muscle listed in the paragraph above, one can get dramatically more air after demanding exercise like sprinting.
Also, the serratus anterior is very involved in the postural support of the ribs. It acts as fingers that extend to the lateral ribs to support the ribs in extending the trunk.
It is a crucial part of stabilizing the scapula. Usually, it assesses as weak in scapular winging.
The anatomy of the serratus anterior has statistically significant anomalies. For example, one study showed seven to ten bellies, with 48% of cadavers having eight bellies.
The upper section varies in its attachment along the scapula. Some cadavers show significantly greater areas of attachment along the superior border than others. Studies show that it attaches to the second rib about half the time.
The intermediate head, which attaches to the vertebral border, is also variable in the number of heads and ribs of attachment. This section attaches just under pec minor, which is also variable in the number of heads and attached ribs. The two work together to protract the scapula from front and back. I have not seen studies indicating that these anomalies match from one cadaver to another.
The lower section is also quite variable in the number of heads and attached ribs. Studies show it extends as low as the 12th rib. Also, studies show it ending as high as rib 6.
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Tony Preston has a practice in Atlanta, Georgia, where he sees clients. He has written and taught about anatomy, trigger points, and cranial therapies since the mid-90s.
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