The muscles of the core

Hi everyone!

I’ve just stumbled upon a great group of videos on the core, from the Aurora Sports Medicine Institute in Aurora, CO.   I wanted to make sure I shared them with you because I thought they presented some really useful information in a way that’s easy to understand.

In the first video, above, Athletic Trainer Molly describes the three muscle groups that make up the core: the rectus abdominis, the obliques, and the transverse abdominis.

As she explains, all of these core muscles work together any time we need to create a movement with our upper or lower body.

Generally, when people think of the abs they think of the first muscle she (and I) are going to explain: the rectus abdominis.  However, there is so much more to the core than that.

I recommend that you first watch the video above, and then check out the notes I took for you below.

The first muscle group in our explanation:

1) The rectus abdominis: The “6-pack” muscle; flexes the trunk.

The rectus abdominis is the most superficial muscle of all the core muscles, meaning it’s the closest to the surface of your body.  This is why, when it’s well developed, it gives you those “six-pack abs” everyone talks about.  This is because it’s the easiest muscle to actually see. 


The rectus abdominis muscle is engaged when you flex the spine, or, in plain English, bend forward at the waist.  This is the main muscle you’re using when you perform the traditional “crunch” exercise.

20 years ago, the rectus abdominis was really the main muscle people thought about when they thought about the core.  That’s why, when I did my mom’s Jane Fonda workout videos for fun as a kid (yes, really!) all of the exercises in the “abs” segment were crunches.

However, we’ve learned a lot more about the core since then, and in particular, the role of the transverse abdominis in stabilizing the spine.  That’s why now, if you go to physical therapy or even to a personal trainer, they probably won’t place the same emphasis on crunches that they used to.  As the video explains, it’s more about how all the muscles of the core work together.  So, moving on to our next muscle group:

2) The obliques (internal and external): side bending and rotation.

The term “oblique” sort of refers to the fact that these muscles are off to the side, and off to a bit of an angle.

Unlike the rectus abdominis, which runs straight up the center of the abdomen, the obliques are located off to the sides of the abdomen.

As the names suggest, the internal obliques run underneath the rib cage, while the external obliques run above it, closer to the surface of the body.


Because they are off to the side, the obliques can perform motions that involve bending to the side, or rotating through the trunk.

This is why, in my Jane Fonda videos, she would talk about the obliques while doing the sort of crunch that would involve bringing one knee towards the shoulder on the opposite side.  It’s not a full rotation through the trunk (and with the SI joints, I really don’t recommend any exercises that do involve turning or twisting!).

Luckily, the obliques can be engaged with more subtle movements that don’t require you to do anything extreme, to fully turn or rotate through the trunk.  (More on specific exercises later!).

3) Transverse abdominis: stabilizes the spine and pelvis, including the SI joints.

The transverse abdominis is the deepest of all the muscles of the core.  It’s also the one we tend to be the least aware of, because we can’t see it.  It doesn’t give us a visible “six-pack” when it is strong.

However, the TA is arguably the most important muscle of all to strengthen, in order to stabilize the SI joints.  Studies have shown that, when it’s contracted, it directly reduces the amount of force that travels across the SI joints during movement (more on that later).

In this video, which is a follow-up video to the first video I posted above, the same athletic trainer (Molly) shows how to locate the TA, and identify its contractions.

First, she recommends that you do a “crunch.”  When you do this, you can see how the muscle contraction actually makes your core muscles lift upwards, into your fingers.  That’s the rectus abdominis contracting.  It is, as I said, bending your trunk forwards.

Meanwhile, when you turn over and follow her instructions for contracting the TA– pulling your belly button away from an ice cube– you actually feel your abdomen lift up towards your spine, as if you’re getting skinnier.

These two exercises are a great way to learn the difference between the rectus abdominis and the TA.  Although they are both major muscles of the core, their functions are extremely different.

Here is a picture of the TA, just so you get an idea.  I think it literally does look like a corset.  When you contract it, it’s like you’re tightening the laces of that corset, stiffening the torso and the spine:

Transversus_abdominis (1).png

So how do we train the abdominal muscles to reduce back pain?  

Many people, myself included, believe the TA is the most important muscle to focus on, in terms of stabilizing the core.  However, as Molly explains in the first video, the end goal is to have all of the abdominal muscles working together, efficiently, when we need them.

So how do we do this?

Luckily, all of the muscles above do play a role in supporting the trunk in the way the transverse abdominis does.  So, when we begin to train the TA in the way Molly demonstrates in the video above, we are also switching on the rectus abdominis and the obliques.

From a physical therapy perspective, we focus on educating people about the TA because it’s the one they are least likely to be aware of, and the one that will have the most direct impact on allowing them to heal.

And it is this action of “compressing” the abdomen (that squeezing in motion, as if you’re trying to withdraw your belly button away from an ice cube) that really stabilizes the spine and pelvis.  This isn’t to say it isn’t important to also be strong in the other motions (the “crunch” motion, or turning motions).

But from a rehabilitation perspective, it’s this “compressing” motion that’s really going to play the greatest role in reducing pain, and getting you up and moving again.

For me personally:

In my own recovery, I learned to activate the TA on land (with a physical therapist– it’s very hard to do on your own).   Then found it came very naturally to me to activate it while I was in the pool.  My lower back would actually start to hurt when I had bad posture or was arching my back, and I’d immediately remember “oh!  Contract the TA!”  And the pain would be gone.  For me, it very directly demonstrated the function of the TA.

However, as I progressed through my aquatic exercises, I did start to do maneuvers that involved the other motions (the “crunch” motion and movements that engaged the obliques in a sideways manner).   I strongly believe it’s easier to strengthen the core– and to strengthen anything else– in the water.

But I did also supplement my pool work by practicing the “drawing in” maneuver on land, and doing the core exercises I’ve referenced in my previous posts on the TA and core strengthening:

So…. you can get a LOT better simply by learning how to contract the TA, and performing this drawing in maneuver.  I honestly didn’t do anything crazy (nothing like some of the wild ab exercises you see on Youtube) before I was able to go my first eight months without having my SI joints lock up.  The TA was really like my secret weapon, and once I figured out how to use it, things got a lot better.

Okay… that’s all for now!

This started out as such a simple post in my head, but (like every post) ended up being a bit more complicated when I wrote it all out.

As always, if you have any questions, feel free to email me at, or leave a comment below!

Some of you may know that I have officially started working on my Sacroiliac Joint Exercise e-book so FINALLY… I’ll be able to adequately answer the question of “What are your exercises?”

Until then, I hope this was helpful!

Muscle Illustration credits:

Rectus abdominis: CC BY-SA 3.0,

Obliques: Public Domain,

Transverse Abdominis:  Modified by Uwe Gille – Gray397.png, Public Domain,

4 thoughts on “The muscles of the core

  1. Ann says:

    So glad I found your blogs. I started with my third physiotherapist today. Have had SI joint dysfunction for a year now. This new physio is the first one who mentioned the TA muscle and I’ve been trying to remember to do it since I read about it on your blog. Looking forward to your e-book.


    • sunlight in winter says:

      Hi Ann, so glad my blogs have been helpful! The TA can be such a powerful tool when you know how to use it, so I’m really glad this physio is working on it with you! And thanks for saying that about my e-book– hoping to have it out soon. 🙂


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