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We often hear yoga poses like side plank and plank pose described as "core stability" exercises. But are they really?

Before we answer this question about core stability, let’s take a step back and examine what the term “stability” in and of itself means.

From a technical standpoint, *stability* can be defined as our ability to maintain or return to equilibrium in the presence of a perturbation.

A good analogy for this phenomenon is to picture a bowl with a marble inside of it. In this setup, if you were to perturb or jostle that marble, it would oscillate from side to side for a time, and then it would eventually return to its original position of resting in the center of the bowl.

In other words, a bowl with a marble in the bottom of it is a stable system because it returns to equilibrium after a perturbation.

On the flipside, picture an upside-down bowl with a marble resting on top of it. This system is inherently unstable because the slightest perturbation would cause the marble to roll to the side and off the bowl. There’s no way it would return to its original position on top of the bowl on its own.

Remember: a stable system maintains or returns to its position (a.k.a. “equilibrium”) in the presence of a perturbation.

So that’s the scientific definition of stability. Now what does that have to do with the human body, and with the movements we do in yoga?!

In a nutshell, stability is synonymous with balance, or “postural stability.” A body that is well-balanced is stable. It’s able to maintain its center of gravity over its base of support after a perturbation. Meanwhile, a body that’s unstable would be more likely to fall over (or take a recovery step, thereby changing position) than return to equilibrium after a perturbation.

What about core stability?

In a classic research paper called “The Role of Core Stability in Athletic Function,” Dr. Ben Kibbler and colleagues define core stability as “the ability to control the position and motion of the trunk over the pelvis to allow optimum production, transfer and control of force and motion to the terminal segment in integrated athletic activities.”

With core stability, the idea is that we’re controlling the position of our trunk so that we can transfer and produce force optimally through this region of the body.

This is an extension of the definition of “stability” as an isolated term that we outlined. Whereas stability describes a system that can maintain its orientation in the face of a perturbation, core stability narrows this focus down to the “system” of the core or trunk region of the body.

In order for an exercise to be considered a core stability exercise, then, it must involve a perturbation. The perturbation is the necessary ingredient that actually challenges our control of our spinal position.

In general, core stability exercises consist of holding our spine in one position while one or more of our limbs move (the perturbations). Classic exercise examples include bird dogs and dead bugs.

Given this definition, many yoga poses we traditionally think of as challenging our core stability don't quite fit the bill.

Take side plank, for example:

In side plank, our side body (obliques and hip abductors) works against gravity to maintain a straight-line position from our head to our heels.

While it's certainly challenging to maintain this position, there isn't much going on in the way of a perturbation.

Nothing from the outside world is jostling us out of position, which would be an *external perturbation*. (Unless we have a particularly cruel yoga teacher! 😛)

Nor are we ourselves resisting spinal movement in the presence of movement of our limbs, which would be an *internal perturbation* – not in traditional side plank, at least.

Enter this dynamic movement: Side Plank with Elbow to Knee:

Our goal in this side plank variation is to maintain our balance in side plank while we tap our elbow to our knee.

Thanks to the arm and leg movement involved, now we DO meet the specifications for an internal perturbation in side plank – and therefore this is a true core stability challenge!

Let's look at another example: plank pose:

Plank pose is certainly a challenge for our anterior core (abdominals).

But because we're not subject to forces that could be classified as external or internal perturbations in this pose, traditional plank isn't a true core stability exercise.

But now consider this variation of plank pose: the Plank Alternating Shoulder Tap!

Here, we alternately lift one hand and tap our opposite shoulder. This limb movement is an internal perturbation. In response, we try to maintain our plank shape.

The Plank Alternating Shoulder Tap is a perfect example of using perturbations to create a core stability challenge.

(Pro tip: You can decrease the challenge involved here by placing your feet farther apart, and you can increase the challenge by bringing them closer together.)

Let's look at a final example in this series: forearm side plank:

As with our regular side plank example from above, traditional forearm side plank doesn't involve much in the way of a perturbation – either an external or internal perturbation.

Instead, our main focus in this pose is to work our side body to maintain our forearm side plank alignment. So there's not much of a core stability challenge involved here!

But it's a different story with the Rolling Forearm Side Plank!

In this move (which is done much more slowly than what I've depicted in this quick gif! 😛), our core is challenged to hold *still* as we rotate our whole body in space.

This rotational movement, which takes place in our "standing" shoulder joint, is an internal perturbation.

We must maintain our core position in the face of this internal perturbation, and so this move is a significant core stability challenge!

Now that you have a sense of the role that perturbations play in stability challenges, you can apply this knowledge and work on core stability in your yoga practice and/or teaching!

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