How the inner and outer unit work in core exercises

In Pilates we almost habitually cue transverse abdominis (TVA) to find core activation and correct deviations in core training exercises. However, what if a lack of TVA isn’t the problem and how can you tell?

To understand this, your need a quick anatomy lesson. You core is a cylinder composed of an inner and an outer unit.

The inner unit stabilizes your spine on a segmental level and holds your guts in. It is made up of your pelvic floor, diaphragm, TVA, multifidus and a few other deep muscles that I’m not listing for simplicity’s sake.

If your core were a house, the pelvic floor would be the ground and the diaphragm would be the roof. The multifidus would be the main support beam and the TVA would be the walls that wrap around the house and attach to the support beam.

While the inner unit is responsible for stability, the outer unit is responsible for moving you. It is what allows you to twist, extend and bend your spine as you move through life.

The outer unit is comprised of your internal and external obliques, erector spinae, lats, glutes, QL, adductors and hamstrings. To return to the house analogy, these muscles would make up the outside of the house. The external obliques, lats and QL would act like siding and the adductors and glutes would be the support beams under the foundation.

So how does this relate to how we move one more important than the other?

To have functional strength and pain free movement, you need both your inner and outer unit to function and work together. If your inner unit isn’t firing well, then your are more likely to lack segmental control (i.e. your spine will move as a solid brick instead of across several vertebrae) and stabilization in the spine even if your outer unit is rock solid.

However, if your outer unit can’t do its job, then you will lack the strength to keep your spine in good alignment under high velocity movements and heavier loads (think running, jumping, weight lifting).

The inner unit should pretty much be on all the time, but in terms of muscle timing, when we prepare to move the TVA should kick on automatically and shortly before the obliques, which will create increased trunk stiffness and/or motion.

Let’s say that the obliques come on, but the TVA doesn’t. An indicator that this has happened would be if you saw your abs push out when you went to move, but your pelvis and ribs stayed relatively stable and aligned.

Conversely, if your abs naturally and gently drew inward, but your pelvis or ribs shifted or twisted when you went to lift a leg or move your arm, then that would mean one or both of the obliques failed to fire correctly during the movement.

However, it’s worth noting that you can have a failure of the inner unit or the outer unit, but it’s often a little of both.

Here’s an example of what this might look like.

In the top video, I’m firing both my inner and outer unit. You’ll see my abs don’t “pop” out when I lift my leg and that my hips stay fairly stable.

In the second video, I’m showing common deviation of failing to fire the inner and outer unit in good timing. My abs are pushing out slightly and there is a marked hip hike when my leg comes up.

So how might we fix this?

If I saw the abs “popping” up on a client, then I might start by cuing a breath pattern. Something like inhale to prepare and exhale to feel the core gently draw in as the leg lifts. However, know that the second the leg lifts, the exercise has become more than just a TVA activation. The outer unit has to come on to lift the leg. Any and all movement requires activation of the outer unit. 

If that looked pretty good, but I still saw a hip hike, then I would cue keeping the pelvis stable. This would be a way to target the obliques and corresponding outer unit without having to cue any one specific muscle, since our core works as a unit and not a set of isolated parts.


Unless you are doing a breathing exercise, it’s impossible to do any movement without using both your inner and outer unit. With this in mind, the question becomes, which unit are we focusing on when we move?

Any time you see shifting or twisting in the pelvis or rib cage or tucking or arching of the pelvis, know that something is going on with the outer unit of the core. If you see abs pushing out, but the ribs and hips are relatively stable, you may want to bring your focus to deep stabilization of the inner unit.

Heel slides, toe taps, leg lifts are lovely, gentle exercises that will let you focus on the inner unit since the load is low and the movement is small. Yes, your outer unit is still working, but the lighter the load, the easier it will be to find that deep stabilization.

The second you add active twisting or load, you’ll need more support from the outer unit. This could be virtually anything load bearing – squats, lunges, planks, rotating planks or bridging – it really doesn’t matter. They’re all going to call for the outer unit to come online. However, if you don’t have a working inner unit, then it wouldn’t be advisable to add heavy load or impact until this has been corrected, since it increases the chances of a back injury.

If you have a real deficit in the inner unit (think pelvic floor dysfunction, chronic back pain, disc problems), you will want to see a physical therapist and then possible work with a trainer who understands post-rehab exercises for improving the muscle timing before you try or return to more heavily loaded exercise. As a Pilates teacher, this is my jam, but there are certainly other modalities that teach this.

As I mentioned before, both units are important, so the real goal should be having good timing with both units, first under low load and then subsequently with more challenging exercises as your progress over time. Do some work with both a local (inner unit) and a global (outer unit) focus and you’ll get the best of both worlds.

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