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by Jenni Rawlings & Travis Pollen, PhD

Have you noticed that “stability” is somewhat of a buzz term in the yoga, movement, and fitness worlds these days? Although this word is commonly referenced, it’s often used in multiple ways to mean different things. As a result, it can be a confusing and misunderstood concept.

In this article, we’ll take a closer look at “stability,” including the technical/biomechanical definition for this term, as well as common contexts in which we hear this word used, such as “core stability,” “spine stability,” and “joint stability.” We’ll also explore the relationship between stability and mobility. We hope to clarify this confusing term so we can be more accurate when we use this word in relation to the body, yoga, and movement.

Also, heads up: We’re excited to share that as a companion to this article we also recorded a podcast in which we cover this nuanced topic in an engaging, digestible, and conversational way. This is the very first episode of our brand new podcast: Yoga Meets Movement Science(!!)

You can dig into the episode wherever you listen to podcasts. If you enjoy our chat, please subscribe, rate, and leave a review! We hope you enjoy this added resource for learning and connecting with us about yoga and movement science.

5 COMMON USES OF THE TERM “STABILITY”

#1: THE BIOMECHANICAL DEFINITION OF STABILITY

When it comes to the scientific realm of biomechanics, physics, and engineering, stability has a singular, clear definition. In this context, a system is considered “stable” when it can return to equilibrium after 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.

(Granted, if you were to jostle the marble with excessive force, it might end up being propelled out of the bowl – but let’s just picture a moderate “flick” of the marble for the purpose of this example. :) )

Now picture the same bowl and marble, but instead of the marble resting inside of the bowl, the bowl is turned upside down. In this case, the marble is resting on top of the bowl, delicately balanced there. If you were to perturb the marble here, even with just a small tap, it would roll to the side, off of the bowl, and perhaps somewhere across the room! There’s no way it would return to its original position on top of the bowl on its own. This is an example of an “unstable” system because it did not return to equilibrium after the perturbation.

So that’s the scientific definition of stability. Now what does that have to do with the human body?!

In a nutshell, this definition of 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.

To consider balance in a yoga setting, picture yourself standing in tadasana (mountain pose) on your yoga mat. You can probably imagine that if you were to stand with your feet hip’s distance apart or even wider than that, this would be a relatively stable position for your body because your “base of support” is rather wide. If someone were to come up and give you a little push from the side, you’d first sway in response to that perturbation then re-settle back into your original tadasana.

But if you were to stand with your feet all the way together so the inner edges were touching, this would narrow your base of support. You’d have less leeway for how far your center of gravity could sway in response to a perturbation and still return to equilibrium afterward.

And what if you were to lift one foot off the floor so that you were standing on one leg (e.g., in tree pose)? That would be even less stable because you’d be removing a point of contact with the floor! Can you imagine being pushed from the side while in tree pose? Instead of maintaining your equilibrium in that single-leg balance, there’s a good chance you’d have to come out of the pose by setting your leg down.

Is “balance” always what people mean when they use the word “stability” in a yoga, movement, and fitness context, though? Most certainly not! Let’s turn our attention to some other common uses of the word “stability” – and see whether they align with the scientific definition of the term.

#2: STABILITY AS A SYNONYM FOR STRENGTH

It’s quite common to hear the words “stability” and “strength” paired together as though they were the same or very similar concepts. While stability is certainly related to strength, these two terms aren’t actually synonymous. To communicate clearly about the body and movement, we want to be sure to understand the difference between these qualities.

The scientific definition of stability is the definition we just covered in the last section: the ability to return to equilibrium after a perturbation. The scientific definition for strength, on the other hand, is that strength is a measure of force production. Whereas a good way to measure (postural) stability would be to see whether someone could stand on one leg for 20 or 30 seconds, a good way to measure strength would be to see how much weight someone could lift.

Although there’s a certain amount of strength required for standing on one leg (especially in the lateral hip muscles of the standing hip!), the movement attribute that generally plays a more dominant role here is balance/stability. And while picking up a heavy weight does involve some level of stability, the movement ability that’s truly playing the starring role here is strength.

Here’s an example of a way in which the concepts of strength and stability are sometimes conflated. There’s a movement pattern we sometimes see in which someone’s knee tracks inward when they run, jump, squat, etc. This pattern is known as knee valgus or “knock knees.”

This movement pattern is sometimes seen as one that should be “corrected,” and the prescription given for correcting it is often to perform exercises to strengthen the lateral hips. The idea is that by strengthening the outer hips, the body will naturally reorganize itself during running, jumping, and squatting to keep the knee tracking straight forward instead of caving inward.

However, it turns out that strengthening the outer hips doesn’t guarantee that someone will automatically change their knee tracking pattern during movement. And that’s because knee tracking isn’t simply a matter of strength! Although there is a certain amount of strength required to track the knee straight forward, the alignment of the knee during movement is actually more a matter of stability than pure strength.

Again, stability can involve a certain amount of strength. But once someone has the necessary strength for a particular movement, what really determines what a movement pattern looks like is how the nervous system coordinates and controls that particular movement (i.e., “motor control”).

In light of these definitions, we can think of strength as having to do with the amount of force that’s produced by the body, whereas we can think of stability as being strength plus control.

With knee valgus, if we only do strength exercises in an attempt to modify this movement pattern, we’re using a strength strategy (force production) to address a stability (control) issue. In reality, once we have the baseline strength for our desired movement, the best way to change the stability of the knee is to work with the movement pattern itself to retrain how the nervous system controls the body during that movement.

Here’s a quick example of one way we can address control when we see knee valgus:

To sum this point up, stability (balance) and strength (force production) are related but distinct qualities. The interplay between them can also be seen in our next common use of the term stability: core stability.

#3: CORE STABILITY

What is core stability and how is it similar to or different from the biomechanical definition of 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.”

Once again, we see stability discussed in terms of both control and strength. With core stability, the idea is that we’re controlling the position of our trunk so that we can transfer and produce force (strength) optimally through this region of the body.

Core stability is, of course, a skill that we can train. The most common way we do so is with isometric exercises, often holding our trunk in one position while moving our arms and legs. Two classic examples are dead bugs (where we lie on our back and hold our spine still as we reach our arms and legs away from our trunk) and bird dogs (where we’re in a hands and knees (quadruped) position and we reach our limbs away from each other).

The term “core stability” aligns with the biomechanical definition of returning to a position after a perturbation. In the case of core stability exercises, the perturbations are created by the movement of our limbs as we hold our spine in one position.

Core stability exercises are also a good example of the connection between stability and strength. With dead bugs, control of the spine is definitely challenged (the stability element). Strength can also be quite challenged here, as any of us who have tried this movement know! It takes a good amount of abdominal strength to keep our low back pressing into the floor as we reach our arm and leg away from us.

From this discussion of core stability, it’s apparent that although strength and stability are distinct qualities, they certainly overlap in many of our movements. This is one reason why it’s so easy to conflate the two qualities!

#4: STABILITY AS “CONTROL THROUGH A RANGE OF MOTION”

A fourth way we commonly see the term stability used is in reference to “control through a range of motion.” This definition of stability tends to be associated with moving into end range positions and is often discussed in terms of active versus passive ranges of motion. In this context, moving through a full range of motion with control (a.k.a. “active flexibility”) is considered to be “stability.”

If you’re familiar with this use of the term stability, you might also be familiar with the term “mobility.” Mobility is used in conversations about end-range positions and is generally defined as “moving with control” or “moving through a full range of motion with control.” There’s just one problem: this is the same definition we just posited for the fourth application of stability. Yikes!

As you can see, when used this way, stability and mobility are basically the same thing. Furthermore, this definition of stability doesn’t really align with the biomechanical definition of the term (returning to equilibrium after a perturbation).

Because there’s already a perfectly good word to describe the quality of moving through a full range of motion with control (“mobility”), we feel that using stability in this way might be more confusing than helpful. For consistency and clarity, we prefer to let “mobility” be the operating word we use for “control of a range of motion” and to use “stability” when we’re talking about maintaining a position in the face of a perturbation.

#5: JOINT STABILITY

Whereas “postural stability” applies the biomechanical definition of stability at the whole-body level, the term “joint stability” applies to the level of a single joint. In this sense, if we’re talking about how much perturbation is required to take a system out of equilibrium, at the individual joint level we’re really talking about dislocating a joint.

Joint dislocations certainly happen, and some areas of the body are more prone to them than others. For example, while our hip joint is an extremely stable structure that rarely dislocates outside of traumatic accidents, our shoulder joint has less inherent stability on a bone structure level. Therefore, our shoulder joint requires a relatively smaller perturbation to dislocate, and shoulder dislocations are more common than hip locations. (Although shoulder dislocations are themselves still relatively rare!)

Thus, discussing joint stability and instability in terms of dislocations is an accurate application of the term “stability.” But in the yoga, movement, and fitness worlds, we often use the term on a joint-specific level more loosely, and in a way that perhaps isn’t so helpful.

For example, it’s not uncommon to hear a practitioner tell a client/student that they have an “unstable spine” (and therefore need core stability exercises). In reality, true spinal instability is a relatively rare spine pathology where there’s excessive motion between vertebrae. Meanwhile, diagnosing people willy nilly with spinal instability can plant unhelpful seeds for that person as far as their beliefs about their body go.

If someone believes they have an “unstable spine” (because their yoga teacher told them so!), they might picture their spine as though it were a precariously-balanced stack of blocks that could crumble at any time in response to the wrong movement. This might prompt them to develop some counterproductive, hypervigilant tendencies and a habit of “protecting” their spine by not allowing it to move. These tendencies can themselves be problematic, potentially contributing to future pain and disability via psychosocial influences on pain. Bottom line: we should avoid language that implies that without stability work, we’ll be left with unstable joints.

Luckily for us, once we take the time to break down the term stability and the many forms and definitions it takes, we can improve our understanding of this complex concept. Once we have a better sense of what we’re talking about, we can begin to use the term in a more clearly-defined and accurate way, which will better serve our students in the long term!

Remember to listen to Episode 1 of the brand new Yoga Meets Movement Science Podcast (!!), where we’ve discussed this whole topic in an engaging, conversational, and informative way. Learning about new ideas through multiple channels (reading and listening!) can be so helpful for reinforcing our understanding and comprehension.

“See” you on the podcast!

About the Co-Author

Travis Pollen is an author, personal trainer, and Exercise Science professor at Thomas Jefferson University in Philadelphia. His research focuses on athletic injury etiology and risk reduction. He’s particularly interested in pre-participation movement screening, return-to-sport testing, and training load and the roles these factors play in injury risk appraisal. In addition to his PhD in Health and Rehabilitation Sciences, Travis holds a master’s degree in Biomechanics and Movement Science and an American record in Paralympic swimming. He’s been a yoga student for over 15 years.

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