Axial vs. Appendicular

In order to have a good understanding of anatomical directional and positional terms, it is first helpful to know the difference between the axial skeleton and the appendicular skeleton. I’m thinking in particular of the terms proximal and distal, I’ll explain those more in a moment.

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The axial skeleton includes the skull and all the cranial bones, the spine, the ribs and the hyoid bone. The hyoid bone is just inferior to the mandible (jaw bone) and it sits anterior to (in front of) the third cervical vertebrae. A little side note- the hyoid is the only bone in the body that doesn’t articulate with another bone. It serves as an attachment site for the suprahyoid and infrahyoid muscles. The small but strong suprahyoids elevate the tongue and open the mouth while the thin and more dainty infrahyoids act as an antagonist (do the opposite). 

The appendicular skeleton is everything else. It’s everything that attaches on to the axial skeleton. Think “appendages”. The pelvis, femur, fibula, tibia and all of the foot bones as well as the scapula, clavicle, humerus, radius, ulna and all of the hand bones are classified as appendicular.  All of those bones are essentially stuck on to the axial skeleton, which is the centermost part of your skeletonWe use the terms proximal and distal to describe the position of one thing to another in relation to the appendicular skeleton. If it’s closer to the appendicular skeleton it’s called proximal. Proximity means “close to” or nearness so you can remember it that way. Distal means distant or away from, so it’s farther away from the appendicular skeleton. 

Here are some examples:
The calcaneus is distal to the patella. The patella is proximal to the calcaneus.
The wrist is distal to the elbow. The elbow is proximal to the wrist.
The phalanges are distal to the metacarpals. The metacarpals are proximal to the phalanges.
The distal end of the femur articulates with the proximal end of the tibia. 

When we lengthen the spine, we are creating axial extension- creating more space between each of the vertebrae in the vertebral column. From either a seated or standing position, imagine your feet or your pelvis getting heavier on the floor while you actively reach the crown of the head toward the ceiling, like you’re balancing a heavy book. You should feel your spine grow a little longer- this is axial extension.

Erector Spinae Group

The erectors are a powerful group of muscles on the back that start on the sacrum and climb the spine and ribs all the way up to the occiput.  Like a lot of muscles, the ESG has the potential to get complicated/confusing really quickly, but I’m here to break it down in a way that hopefully makes a lot of sense to you. 

Firstly, we’re looking at three muscles here: spinalis, longissumus and iliocostalis. Spinalis is closest to the spine, or the most medial of the three. It climbs the spinous processes. Just lateral to the spinalis is the longissumus, the middle of the three erectors. It climbs the transverse processes. The most lateral of the three is the iliocostalis. If you really think about the name of that muscle it will tell you exactly where it is- on the ilium and the ribs (costal means rib). The iliocostalis climbs the ribs. 

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Let’s talk about the commonalities. 

Spinalis, longissimus and iliocostalis all work together to produce the same movements. When the fibers on the right contract, the spine laterally flexes to the right. When the fibers on the left contract, the spine laterally flexes to the left. Lateral flexion is a side bend. When all of the fibers work together and contract at the same time, the spine extends. The upper fibers of these three muscles also assist in laterally flexing and rotating the head and neck to the same side, as well as extending the head and neck. 

In addition to sharing the same actions, they also share a point of origin on the thoracolumbar aponeurosis, also known as the common tendon. The thoracolumbar aponeurosis is a broad, thick, diamond-shaped tendon that goes from the sacrum over to the ilium and then up the spinous processes of the lumbar vertebrae to the lower thoracic vertebrae.  

Let’s look at them individually, starting with the medial spinalis. The spinalis muscle originates on the common tendon (spinous processes of the upper lumbar and lower thoracic vertebrae) and the ligamentum nuchae as well as on the spinous process of C-7. The ligamentum nuchae is the big, thick neck ligament. 

Doesn’t that seem weird that it starts in one spot, skips over a spot in the middle and then starts again somewhere higher up? 

Yes!

This works because the muscle inserts on the spaces in between. In this case the insertion is on the spinous processes of the upper thoracic vertebrae (this is between the two points of origin) and then also on all of the spinous processes of all of the cervical vertebrae (except for C-7, because the muscle originates there). To me, it feels less important that you remember exactly where the origin/insertion is and more important that you remember the muscle climbs the spinous processes and pulls the spine backward into extension. 

The middle muscle is called longissimus. It’s the thickest and, hence its name, the longest of the three. It originates on the common tendon as well as the transverse processes of the first five thoracic vertebrae (T-1 through T-5). The spinalis climbs the spinous processes while the longissimus climbs the transverse processes. Like the spinalis, the longissimus also does that thing where it starts, skips some space and then starts again. It inserts itself between the two points of origin on the lower 9 ribs (ribs 4-12), the transverse processes of all of the cervical vertebrae (C-1 through C-7) and the mastoid process of the temporal bone, which is behind the ear. You can easily feel this one on yourself or someone else, it’s about two inches wide and its cable-y fibers are on either side of the spine. 

If you don’t remember anything else about the longissimus, remember that its the middle erector as well as the longest, thickest and most easily palpable.

Iliocostalis is the rib climber. It originates on the common tendon and the posterior surfaces of all the ribs (1-12). To get over to the ribs, it inserts on the transverse processes of the first three lumbar vertebrae, then climbs the posterior surfaces of all the ribs to get the the transverse processes of the lower cervical vertebrae. 

Bhujangasana (cobra pose) is a great way to strengthen these commonly tight but overstretched muscles. I like to practice and teach this one with strong legs and little to no weight in the hands to really get these muscles firing. 



Origin vs. Insertion

When talking about muscular attachments, it’s important to know and understand the difference between the origin and insertion. They are not interchangeable and have totally different meanings, though you can say muscular attachment or attachment site and be talking about either the origin or the insertion. 

What’s the difference? 
When a muscle contracts, the origin pulls the insertion closer.  Always! Muscles pull. The origin is the fixed point that doesn’t move during contraction, while the insertion does move. Your bones are the levers and your muscles are the pulley. Basically, we are all super complex puppets on strings. To explore this further, let’s take a look at the rhomboids. 

Rhomboids Minor and Major

Rhomboids Minor and Major

Firstly, there are two rhomboids and they’re both deep to the multidirectional fibers of the trapezius. Rhomboid minor sits on top of, or superior to, its larger counterpart rhomboid major. Minor originates on the spinous processes of C7 and T1 while major originates on the spinous processes of T2-T5. You can pretty easily feel for T1 on yourself, it’s just at the base of the posterior neck and it bumps out a little bit posteriorly. The transverse processes stick out to the sides while the spinous processes, which we are talking about here, stick out and a bit downward posteriorly. They remind me of a stegosaurus. It’s from these little posterior projections that both rhomboids originate. 

Back to rhomboid minor, it inserts on the upper portion of the medial border of the scapula, across from the spine of the scapula. The insertion for levator scapula is right there too and is easily palpable on yourself or someone else. Rhomboid major inserts on the medial border of the scapula as well, basically between the spine of the scapula and the inferior angle of the scapula. The actions of the rhomboids are to retract or adduct the scapula, elevate the scapula and also downwardly rotate the scapula.

WHOA, WAIT.
How does a muscle that elevates the scapula downwardly rotate it as well? If you look at the direction the fibers run, they’re on a bit of a downward angle going from the spine to the scapula. Because of this, when the muscles contract, they have the pulling capacity to draw the shoulder blades closer together. Downward rotation can be a tricky one because the scapula is the only bone in the body to do it, but we’re referring more specifically to the movement of the acromion, which is the flat bony process at the lateral end of the posterior scapula. When the rhomboids contract, they have the ability to tilt the acromion downward. It’s a slight action but it’s there nonetheless. Lastly the rhomboids can also elevate the scapula as a whole. Every time the muscle contracts (concentrically, that is, but that’s another post for another day), the origin says ‘come to me’, and so the insertion does. This is true for all the muscles in the body.

When studying anatomy, learn to look closely at the direction of the muscle fibers, and determine which end is the origin and which end is the insertion. That information will take you a long way in figuring out what actions a muscle can do.

ANOTHER EXAMPLE
Let’s look at a neck muscle- the sternocleidomastoid, or SCM. It has two heads which means it originates in two spots: the top of the manubrium (sternum) and the medial third of the clavicle. Its insertion is up behind the ear on the mastoid process and the outer portion of the occiput. When both the left and right right SCM contract together, the origin (which is down on the clavicle and sternum) pulls the insertion toward it to create neck flexion and also assist to lift the rib cage to make more space for inhalation. 

My hope is that this makes sense but if you don’t understand and would like to, please send me an email and I’ll break it down even more! If you’re teaching and get confused about what’s the origin and what’s the insertion, you can simply say attachment. But do trust yourself! If you take a moment to think about how the body moves and works, and you know the origin is fixed and the insertion moves, you can likely figure it out for yourself.