Obliquus Capitis Inferior - A Misnamed Muscle

Obliquus capitis inferior. Permission Dr. Joe Muscolino (learnmuscles.com).

Key Takeaways

The OCI is one of the four suboccipital muscles, yet it does not attach onto the head despite having “capitis” in its name.
Muscle names usually encode free information — location, attachments, action, shape, or related muscles — which is why misnaming matters.
The OCI runs from the spinous process of the axis (C2) to the transverse process of the atlas (C1).
It produces ipsilateral (same-side) rotation of the head at the atlanto-axial joint, accounting for nearly 50% of all cervicocranial rotation.
The only other misnamed skeletal muscle in the body is the iliococcygeus portion of the levator ani.

In This Article

Introduction

The obliquus capitis inferior (OCI) is one of the four suboccipital muscles. It attaches from the spinous process of the axis (C2) to the transverse process of the atlas (C1), therefore it does not attach onto the head, so it should not have the word capitis in its name, so it is a rare example of a skeletal muscle that is misnamed.

Does Misnaming Matter?

Yes! Definitely! As a general rule, when studying the muscles of the body, we need to learn their attachments, joint actions (functions), and sometimes their nerve innervation. This can be quite intimidating for a student given that some 100+ muscles need to be learned.

So, when a student first approaches learning a new muscle of the body, looking at the name is the first thing that should be done because it offers some free information about that muscle; and each piece of free information is one fewer piece that needs to be learned (perhaps memorized). The name of the muscle might tell us something about its location, specific attachments, joint action functions, shape, or even the presence of other related muscles.

For example, the muscle name biceps brachii tells us that it muscle overlies the brachium, in other words, the (upper) arm.

Or it might tell us something about the muscle’s actual bony attachments. For example, the coracobrachialis attaches onto the coracoid process of the scapula and the arm. A further example is that the other three suboccipital muscles, the rectus capitis posterior major, rectus capitis posterior minor, and obliquus capitis superior, all have capitis in their names and therefore attach onto the head.

Muscle names often tell us about the muscle’s joint actions. The flexor carpi ulnaris tells us that this muscle flexes and ulnar deviates the (hand at the) wrist joint. Flexor for flexion and ulnaris for ulnar deviation; and carpi means of the wrist.

And we might have information about the shape of the muscle; the quadratus lumborum is four-sided (quadratus means four-sided); the teres major is round (teres means round).

In fact, given that qualifiers in the name of a muscle are placed only when needed, the name can also tell us about the presence of other related muscles. In the case of the previously mentioned flexor carpi ulnaris, the word flexor would not have been included if there were not an extensor carpi ulnaris; nor would the word ulnaris have been added had there not been a flexor carpi radialis. In this case, the name of this muscle told us about the existence of these two other related muscles, even if we had not yet learned them.

And to follow a previous example, because the teres major has major in its name, there must be a teres minor (a smaller round-shaped muscle). And the fact that the previously mentioned quadratus lumborum has lumborum in its name, we know there must be another four-sided muscle in the body; in fact, there are more than one; quadratus femoris and the pronator quadratus are two examples of others. And the fact that our featured muscle for this article is the obliquus capitis inferior tells us that there must be an obliquus capitis superior (at least inferior part of the name is correct and helpful).

For this reason, being able to look at the name of a muscle, and trusting that the name has been appropriately given, is extremely important. If muscles can be misnamed, then we cannot trust whatever clue(s) its name has seemingly given us. Without this trust, we are left having to learn (and perhaps memorize) much more about the muscle, which is certainly more work. Therefore, it is a very good thing that misnamed skeletal muscles are so rare in the body. Other than the obliquus capitis inferior, the only other misnamed skeletal muscle is the iliococcygeus of the levator ani muscle (a muscle of the pelvic floor); it is misnamed because it does not attach onto the ilium.

Obliquus Capitis Inferior Attachments

Obliquus capitis inferior. Permission Dr. Joe Muscolino (learnmuscles.com).

The OCI attaches from the axis (C2) (its origin) to the atlas (C1) (its insertion).

More specifically, the OCI attaches from the…

spinous process of the axis (C2)

to the

transverse process of the atlas (C1)

Obliquus Capitis Inferior Functions

The OCI is an extremely important muscle for rotation of the cervicocranial region. It runs quite horizontally in the transverse plane, crossing the joint between the atlas and the axis (atlanto-axial joint/AAJ), which is a uniaxial pivot joint that is responsible for nearly 50% of all cervicocranial (head-neck) rotation. So the OCI rotates the atlas at the AAJ. Because the atlanto-occipital joint between the atlas and the head allows little to no rotation, if the atlas rotates, the head will go along for the ride and rotate with it. Therefore, the OCI can be said to rotate the head.

Pulling the more-superior atlas attachment toward the more-inferior axis attachment results in transverse-plane ipsilateral (same-side) rotation of the atlas on the axis at the AAJ, in effect ipsilateral rotation of the head.

Nearby Anatomy

The suboccipital group. Permission Dr. Joe Muscolino (learnmuscles.com).

As stated, the OCI is one of the four suboccipital muscles. The other three are the obliquus capitis superior (OCS), rectus capitis posterior major (RCPMaj), and rectus capitis posterior minor (RCPMin). Of these, the RCPMaj attaches onto the spinous process of the axis along with the OCI.

The OCI lies deep to the semispinalis capitis and splenius capitis muscles. A small amount of the semispinalis cervicis, multifidus, and rotators lies deep to the OCI.

Palpating the Obliquus Capitis Inferior

General location for palpation of the suboccipital group. Permission Dr. Joe Muscolino – The Muscle and Bone Palpation Manual 3ed.

Because the OCI is so deep (indeed, the more-superficial semispinalis capitis is the thickest muscle in the posterior neck), it can be challenging to palpate and discern the OCI from adjacent myofascial tissue. To palpate it, sit at the head of the table with the client supine. Drop immediately off the midline of the occiput and keep palpating inferiorly until you find the large spinous process of the axis. Once located, drop immediately lateral (and a very tiny bit inferior so that you are not on the nearby RCPMaj) and you will be over the OCI.

Whenever a muscle has a length to it, it is usually best to strum perpendicularly across it. So given the orientation of the OCI, strum from inferolateral to superomedial across it.

Usually, the OCI is palpated with the client relaxed, but it might be helpful to ask the client to gently try to ipsilaterally rotate their head against your resistance and feel for the OCI to engage and harden. Once located, continue palpating the OCI in baby steps toward its transverse process attachment on the atlas.

Stretching the Obliquus Capitis Inferior

Stretching the obliquus capitis inferior (images B and D). Permission Dr. Joe Muscolino – The Muscle and Bone Palpation Manual 3ed.

Given that the OCI is an ipsilateral rotator of the head, it is best stretched with contralateral rotation. So, for example, if you are stretching the right-side OCI, bring the client’s head into left rotation.

Massaging the Obliquus Capitis Inferior

Suboccipital muscle trigger points and their referral zones. Permission Dr. Joe Muscolino – The Muscle and Bone Palpation Manual 3ed.

Massaging the OCI follows from its palpation. Once located with palpation, you can massage it with whatever strokes you like. Given that the palpation protocol is most likely successful with cross-fiber (perpendicular) strokes, these can be performed for the massage treatment. Work from inferolateral to superomedial across the muscle, and back. Longitudinal strokes along the length of the muscle, from inferomedial to superolateral, can also be effectively performed.

Conclusion

The emphasis of this article is on the importance of the naming of muscles, highlighted by the fact that the obliquus capitis inferior (OCI) is misnamed. However, the OCI is an important muscle in its own right. Given the prevalence of locked-short musculature in the posterior neck, especially in the suboccipital (upper neck) region, the OCI is extremely important clinically, therefore therapists should be familiar with its assessment and treatment.

Frequently Asked Questions

Why is the obliquus capitis inferior (OCI) considered a misnamed muscle? The OCI is misnamed because, despite having capitis in its name, it does not actually attach onto the head. Instead, it runs entirely between two vertebrae — from the spinous process of the axis (C2) to the transverse process of the atlas (C1).

What are the attachments of the obliquus capitis inferior? The OCI attaches inferiorly onto the spinous process of the axis (C2) and attaches superiorly onto the transverse process of the atlas (C1).

What is the main function of the obliquus capitis inferior? The OCI produces ipsilateral (same-side) rotation of the head by acting across the atlanto-axial joint, which is responsible for nearly 50% of all cervicocranial rotation.

How do you palpate the obliquus capitis inferior? With the client supine, locate the spinous process of the axis just off the midline of the occiput, then drop laterally and ever so slightly inferior (avoiding the rectus capitis posterior major) to reach the OCI, strumming perpendicular to its fibers from inferolateral to superomedial.

How is the obliquus capitis inferior stretched? Because the OCI is an ipsilateral rotator of the head, it is stretched with contralateral rotation — for example, rotating the head to the left to stretch the right-side OCI.

Is the obliquus capitis inferior the only misnamed muscle in the body? No. The only other misnamed skeletal muscle is the iliococcygeus portion of the levator ani, which is misnamed because it does not actually attach onto the ilium.

Biography

Dr. Joe Muscolino

Dr. Joseph Muscolino, DC is a soft-tissue oriented chiropractic physician and leading educator in manual and movement therapy. He is the author of eight major textbooks published by Elsevier and LWW, translated into more than 10 languages and used worldwide in core curriculum and clinical practice. A global lecturer and NCBTMB-approved CE provider, he offers COMT (Clinical Orthopedic Manual Therapy) certification workshops across the US and internationally. Visit his website at: LearnMuscles.com.

LearnMuscles Continuing Education (LMCE) is one of his online subscription platforms with over 4,000 video lessons for manual and movement therapy professionals, and more than 320 free NCBTMB-CE hours.