Perhaps the most common understanding of stability is “not moving” or “stillness.” In yoga, the idea is often that in order to be stable a body part should be prevented from moving. When we apply this notion of stability to our yoga practice, we are inclined to hold parts of our body rigid. Some examples of this “stability as rigidity” strategy are the common practice of drawing the lower abdominal in throughout a yoga practice, the emphasis on non-moving, protracted shoulder blades during the movement into chaturanga dandasana, and the belief in the importance of a “neutral spine” that neither flexes nor extends in the majority of our yoga poses.
But despite the widespread understanding of “stability” as meaning “unmoving,” this is not actually the true textbook definition of the word. In kinesiology (the study of how the body moves), stability is technically defined as how well a system can return to an orientation after a perturbation. For example, picture a yoga student in side plank (vasisthasana) who is strong and actively engaging through her bottom arm. If someone were to come up and bump this student (i.e., initiate a “perturbation”), her body would move somewhat in response to the bump, but then it would most likely quickly return to its original side plank position with little disturbance. This is an example of a stable side plank. “Stability” in this sense doesn’t mean that no movement happened—it means that when an unexpected bump happened, the student had control over her position and could return to her side plank shape efficiently.
Now picture this same yoga student in side plank, but this time her bottom arm is not very active, and instead of using muscular engagement, she has “locked out” her elbow joint and is leaning into the ligaments of that joint. If someone were to bump her in this version of side plank, it’s likely that she would fall to the floor, because her arm was simply propping her up in the pose, rather than being strong and contributing to the stability of the shape. This is an example of an unstable side plank—after a perturbation, the system could not return to its original orientation.
We can therefore consider stability as being less about rigidity and more about having control over one’s movement.
The Difference Between Mobility and Flexibility
Now let’s turn our attention to the concepts of mobility and flexibility. Although these two terms are often used interchangeably, they actually mean different things! There are two main sets of definitions for these terms in the field of kinesiology. In the first version, flexibility has to do with the extensibility of the muscles and other soft tissues that cross a joint (e.g., “flexible hamstrings”), whereas mobility has to do with how the joint itself and its associated structures move (e.g., “mobile hip joint”).
The second version of mobility/flexibility definitions is more relevant to us in terms of yoga. In this version of definitions, flexibility refers to how far a joint can move, whereas mobility is “the ability of an individual to initiate, control, or sustain active movements of the body to perform motor tasks.” [Ref] In other words, flexibility is simply a matter of range of motion (e.g., “How far did the shoulder move?”), whereas with mobility, we look at a range of motion and determine whether someone has the ability to actively control their body within that range (e.g., “In this stretched position, can the person contract their muscles with a significant amount of force—or do they have no strength there?”).
Put another way, mobility has to do with movement (as one might ascertain from its name), while flexibility is more about a static position of a joint—something that can be captured in a still photograph. Consider hanumanasana, yoga’s forward split pose. In hanumanasana, how low you can get your pelvis toward the floor is a measure of how “flexible” you are. However, this static position doesn’t tell us much about how well you can actually move. The majority of yogis who can come into full hanumanasana, resting their hips on the floor, are certainly very flexible, but they may not actually be very mobile. Their bodies can move passively into this position with the help of gravity, but they cannot generate much force in their muscles, if any, while that deep into their joints’ range of motion. In other words, they do not have active control over this position. Instead they can only sit there, leaning on their ligaments and other connective tissues to hold them up. Someone with a high degree of mobility, on the other hand, could slide into and out of hanumanasana without their hands on the floor to help them—using only the strength and control of their leg muscles. (And how many yogis do you know who can do that?)
To summarize, flexibility is purely a matter of distance (How far can the body move?), while mobility is a matter of neurological control and strength (How well can the body actively move itself within that range of motion?).
For many years there has been a widespread assumption in our culture that flexibility is a quality we should pursue because it reduces our risk of injury and generally makes our bodies healthier. Now that we have a better working understanding of flexibility versus mobility, we are realizing that it is actually mobility, not flexibility, that decreases injury and increases joint health and resiliency. In fact, there is a significant lack of scientific evidence to support the notion that increased flexibility decreases injuries; numerous scientific studies have actually concluded that there is no correlation between stretching for flexibility and injury-prevention. [Ref, Ref, Ref]
If we are interested in building healthier, more resilient bodies through our yoga practice, we should be working on mobility, rather than flexibility, in our asanas.
Mobility, on the other hand, is a known means for decreasing injuries and increasing tissue health. The more strength and control that we have available to us in all of our ranges of motion, the better prepared our body will be to catch itself in an unexpected fall, for example, and the stronger and less injury-prone our tissues will be when we enter deeper ranges of motion, such as yoga’s hanumanasana. If we are interested in building healthier, more resilient bodies through our yoga practice, we should be working on mobility, rather than flexibility, in our asanas.
Here are two practical examples to demonstrate the difference. When we interlace our fingers behind us and lift our arms in yoga, as we often do in shalabhasana (locust pose) or prasarita padottanasana (standing wide-legged forward bend), this is a flexibility stretch for the shoulders. That’s because the interlaced fingers basically “prop” the arms in this position without the shoulders having to do much work. If, however, we come into this interlaced position, and then keep our hands as close together as possible but release the interlacing of the fingers—now our shoulders are suddenly working to hold themselves in this same position. This is now a mobility stretch, which is developing active control and tissue resilience in the shoulders’ range of motion.
Let’s return to the example of hanumanasana (forward splits). When we work on this pose the way we normally do, by sinking our pelvis toward the floor without any active work in our legs, it’s a flexibility stretch. We aren’t teaching our body how to control itself in this position—it is simply passively pushing toward our end range. But if instead we were to put the front heel on a yoga blanket and work to slide that foot forward (without our hands on the floor to help!), moving the foot only as far forward as we had the strength to control the movement, and then slowly sliding the foot back toward us again—this would be an example of working on mobility in hanumanasana. This approach to the pose would train our active control of the position, and because of the muscle contractions involved, the tissues of our joints would be signaled to grow stronger and more resilient—all of which contributes to a healthier body, reduced risk of injuries, and better quality of movement.
Mobility and Stability
Remember, flexibility is a matter of how far joints can move, and mobility is a matter of how much control and strength there is when moving into and holding those positions. Now recall our definition of stability, which is the ability of a system to return to an orientation after a perturbation—in other words, how much control over a movement there is. The surprising reality is that stability and mobility have extremely similar definitions—so similar, in fact, that some people consider them to be the same thing! If you are working on joint mobility by building strength and control through a range of motion, then by definition, you are also working on stability. The body is stable when it can control its position.
This brings us to the topic of people who have been told that they are “hypermobile.” People with hypermobility have joints that can move beyond a normal range of motion. Hypermobile people generally do not have control over their excessive ranges of motion, and they are often referred to as having “unstable” joints. Within the context of our second set of definitions, the term “hypermobile” would be a misnomer, however. As we’ve discussed, “mobility” refers to ranges of motion over which we have control. When a hypermobile person moves into a range that is beyond her ability to control, this is by definition no longer mobility—this is flexibility. A more accurate term for hypermobile people, therefore, is “hyperflexible.” In a perfect world where we have clarified all of our terms, those with hypermobility would be referred to as having hyperflexibility instead.
Hypermobile/hyperflexible people are often cautioned against practicing yoga because there tends to be so much emphasis on flexibility in most yoga classes, which is the last thing their bodies need more of. But if more yoga teachers focused on mobility over flexibility, yoga would actually be very beneficial for those with hypermobility. This would require a decreased emphasis on visiting unstable end ranges in yoga classes and an increased emphasis on building strength and active control within currently available ranges of motion. This would allow hypermobile people to continue doing the yoga practice that they love while creating the important stability that their bodies need.
The Importance of Clarifying Concepts
We use a variety of words to talk about the body in yoga. The better we understand the concepts they describe, then the more effectively we can apply these concepts on the yoga mat. On the surface, the terms stability and mobility seem like they could be opposites. But once we clarify their definitions, we understand that they are actually so similar that they could almost be the same concept. Flexibility and mobility, on the other hand, are two words that on the surface seem as though they could be synonyms. But once we clarify their definitions, we understand that they are actually quite different, and that a working knowledge of their difference has the potential to change the way we practice and teach yoga.