The Flexible Definition of Mobility

imageThe CrossFit Journal recently published an article by physical therapists Zachary Long and Brian Casto entitled “The Optimal Shoulder.” The article discusses the shoulder joint as it relates to performance and safety, and offers visuals and descriptions of a variety of rehabilitative exercises. Interestingly, the article uses the term mobility frequently, yet doesn’t use the term flexibility even once.

This reminded me of a question that I have asked myself dozens of times over the last few years: what does the term mobility mean?

Mobility is Everything

Over the past few years, I’ve made an effort to ask this question to all of the CrossFit trainers and athletes that have crossed my path. I have never gotten the same answer twice. This surprised me. I even posted the question on a number of public forums online, but asking more people only led to more responses.

Here are just a few examples of the myriad of answers this question got when I asked “What is mobility” in a few online forums:

“mobility is health”

“mobility refers to a good range of motion”

“The ability to move”

“Mobility is range of motion, lack of pain during movement at edges of range of motion.”

“You just know it when you see it”

“Mobility is art. The perfect expression of human capacity.”

“Having functional strength and being able to do your day to day activities effectively!!”

“The proficiency to organize and accomplish the act of moving.”

My concern is not with the individual inadequacies of these definitions. I’m not an expert in mobility, and don’t claim to know the correct meaning of the term. The issue I have with these answers is more fundamental: Why does a simple survey asking for the definition of a term virtually everyone uses (myself included) elicit such an enormous range of responses?

These answers could all be wrong, but because some of them contradict each other, they can’t all be right either. Surely there is a single, unique, measurable definition of the term. But if there is, why does it seem like none of us have any idea what we are talking about?

As CrossFit trainers, we treat mobility as something that can be objectively measured, like speed or power. The practice of observing mobility before and after mobility-enhancing drills suggests as much. But if we are measuring a capacity (even with the naked eye) we have already defined it. The simple fact that we are able to take such a measurement means you have committed to a quantifiable definition.

For example, speed is measured by dividing distance traveled by time, and its definition, or the question “what is speed?” would be answered by that same formula.

Measuring Mobility, or Flexibility?

So how do we measure mobility? Interestingly, when I ask the same groups of people this question, I get a virtual consensus in the response – Mobility is measured as (and therefore defined as) the range of motion (ROM) of a joint. And herein lies the fundamental problem with the term – ROM of a joint is the commonly accepted definition of flexibility. (CFJ) (Alter, Science of Flexibility, pg.3) (ACSM

At this point, flexibility and mobility seem to have identical meanings, and yet many mobility experts still hold that mobility and flexibility are entirely different concepts. How do they defend this distinction?

Bill Hartman has been cited as defining mobility as “ROM under specific circumstances”, and flexibility as “range of motion about a joint non-specifically.”

What does this mean? In an article on the IYCA website, Mike Robertson defends this view, explaining that mobility might refer to the ROM of a joint in a specific movement, like a squat, while flexibility would be something like “…you lay someone on their back and stretch their hamstrings.”

But this movement, though perhaps less complex, is equally specific. What Robertson seems to be implying is that the term flexibility becomes mobility only when talking about a movement he cares about. This distinction is neither objective nor necessary.

Another interpretation of Hartman’s definition of mobility is that a joint is “mobile” when a person can achieve the ROM requirements for a particular movement, like a squat. Flexibility, on the other hand refers only to a static tests of joint ROM.

But this simply isn’t how the term flexibility has ever been used. Michael J. Alter, author of The Science of Flexibility, notes that while the definition of flexibility has been debated, movement has always been a component of the term. He writes that for some,

“…flexibility also implies freedom to move (Goldthwait 1941; Metheny 1952), the ability to engage a part of parts of the body in a wide range of purposeful movements at the required speed (Galley and Foster 1987)…and the ability to move a joint through normal ROM without undue stress to the musculotendinous unit (Chandler et al. 1990).”

To anyone who regularly sits down and stands up, the squat would certainly constitute a “normal range of motion”, and would therefore be an acceptable test for flexibility. This means that Hartman’s distinction between mobility and flexibility is not only subjective, but is contradicted by the widely acknowledged use of the term flexibility going back to the first half of the 20th century.

Editors of the TRX blog  have a slightly different view of the issue. They first define flexibility as “…the total available range of motion [of a joint].”  They go on to define mobility as

“…how well you can move through a range of motion. In essence, mobility capitalizes on your flexibility. Being able to use your full range of motion while performing a strength building exercise first requires flexibility, and then mobility is used to move the load through your full range of motion utilizing strength.”

In other words, TRX believes that mobility is a mysterious force athletes use to move external loads through the range of his or her flexibility, a force independent of strength. Here we have the term mobility filling a gap that doesn’t exist. A measure of the ROM of a joint is flexibility, and the measure of how much load a person moves in the completion of a particular task is strength. What part of this physiological model has not been adequately addressed by these two terms?

Some mobility experts tie mobility to the concept of stability. One trainer I spoke to even defined mobility as “The love child of flexibility and stability.” But both flexibility and joint stability are unique terms that can be measured in specific ways. Flexibility is measured by the ROM of a joint, and stability is an assessment (usually) of the overall ROM of the joint and how close to average that is, with an excessive or abnormal ROM leading to a diagnosis of instability (Alter, Science of Flexibility pg. 4).

I looked harder, and eventually came across an article posted by physical therapists Daniel Brownridge and David Larson. In the article, flexibility is defined as “…the ability of a muscle and other soft tissues to lengthen,” while mobility is defined as “…the degree of movement occurring at a specific joint.”

While this might seem like a meaningful distinction, the author goes on to explain that muscle flexibility (or lack thereof) should be assessed by measuring joint mobility. In other words, the author is acknowledging that joint ROM cannot be assessed independently of the tissues surrounding that joint, and flexibility and mobility are again reduced to identical measures.

Interestingly, the article goes on to explain that mobility can be tested by “passively moving the joint” – something that would clearly qualify as non-specific ROM (and therefore a measure of flexibility) based on Hartman’s definition.

A number of other mobility experts I came across seemed to equate flexibility with static stretching, and go on to argue that improving the ROM of a joint requires not just training flexibility, but also mobility. While this is clearly a confusion of terms, it does explain the passionate opposition one can find to the notion that flexibility and mobility are the same thing: some people just don’t like static stretching.

At this point, I was beginning to wonder if the question “what does mobility mean” shouldn’t be replaced with “what doesn’t mobility mean?

image

I decided to look into published, peer-reviewed research papers on the subject.

As it turns out, a brief search of Pubmed yields dozens of academic studies in which scientists measured flexibility as the ROM of a joint using a device known as a goniometer (here, here, and here to identify a few). And yet it is just as easy to find academic studies in which mobility was measured by the ROM of a joint using the goniometer (here and here). Even at the highest levels of academia, mobility and flexibility are treated as functionally identical terms.

Does it matter that no one can agree upon the definition of Mobility?

I say yes. When a term means something different to each person who hears it, poor communication results. In the pursuit of the advancement of human fitness and health, accuracy and consistency of terms matter. Trying to measurably improve a capacity that you can’t define is like groping around in the dark for a light switch.

It seems to me that in order to communicate clearly, we have two options; acknowledge that mobility is just another word for flexibility, or define mobility in such a way that a distinction between the two terms is warranted.

22 comments

  1. It would be interesting to go back to the same forums and ask, “What is fitness?” I think one of Glassman’s key achievements was to establish a clear definition of the term when organizations like the ACSM offered up either fuzzy definitions or definitions that seemed to be created to appease all the various players in the health and fitness marketplace. As Glassman has put it, “If you and I want to know how many bugs there are in the cornfield, first of all we have to come to an agreement on what a bug is.” (I think the allegory was bugs in a field; something like that). At any rate, in CrossFit, we have a clear definition of what fitness is and it’s defined in a way that it can be measured and graphed, and–of course—Glassman also offers that if you plot your fitness data over time you have a 3D graph of what your health is. This definition of fitness has been central to the Level One seminar since the first one. As much sense these definitions of fitness and health make, they have yet to be adopted outside of the CrossFit world (to my knowledge anyway). As far as ‘mobility’ and/or ‘flexibility,’ I agree with you: cutting through any confusion or competing definitions would be helpful. What gets measured gets managed, and it starts in agreeing on what a bug is. At the CF seminar I attended about a year ago, I don’t recall any specific teaching on what constitutes either ‘flexibility’ or ‘mobility’—although in one of the exercise demonstrations (I think it was the overhead squat), the term “hyper-mobility” was used when the coach demonstrating the movement displayed a range of motion with the shoulders that had dropped just about everyone’s jaw–it had those of us in the audience thinking about our own levels of shoulder mobility. I seem to recall it was Matt Chan who brought out the usage of ‘hyper-mobility’ and he didn’t have to explain it—we all knew what he meant. In fact, if had had used the word flexibility it wouldn’t have felt as accurate. Perhaps certain textbooks would define flexibility and mobility as the same thing, but that’s one of the quirks of language—as a group, when it comes to performing certain exercises that reveal the range of motion (or lack-there-of) in a joint, in a CrossFit gym mobility is the word that’s most often being used (in my experience, that is). The only thing I really have to offer to the discussion is that for me, the use and emphasis on “mobilizing” as opposed to “stretching” has been valuable. I come from the era where flexibility was some sort of vague expression of “loose muscles” and the result of stretching the hamstrings or quads. Seems we wasted a lot of time in the warm-up period of football and track practice half-heartedly performing the stretching routine we were taught. We certainly never increased our ‘flexibility.’ And neither the coaches or players would have been able to cough up a meaningful definition of what flexibility was. As far as the present, I would suggest that the word mobility has been valuable for older CrossFitters like myself because we learn one way or another that attending to the joints and the tissues surrounding the joints is helpful in being able to move in the way that a Coach Burgener would insist we do. So, as it was with the word ‘fitness,’ it seems that CrossFit can put forward whatever definition it wants to, and I imagine this would either come from Glassman or Starrett or both. If they haven’t already at some point.

  2. Andreo Spina has defined “mobility” as active ROM at a joint and “flexibility” as passive ROM at a joint.

    I find this to be a relevant distinction, and I think this captures a lot of what people like Robertson are trying to say.

      • Right on. The dynamic, static-active, static-passive distinction makes sense and I agree that covers the relevant differences. Not sure if I see a problem with someone talking about mobility vs flexibility instead of static-active flexibility vs static-passive flexibility, as long as they’ve defined their terms, though.

        The FMS/SFMA folks also differentiate between tissue extensibility/joint extensibility dysfunction and stability/motor control dysfunction when encountering limited range at a joint. Comparing how a joint performs in static-active vs static-passive ROM can give some insights here.

        However, once you start to really dig into how the CNS controls ROM (especially via PRI & DNS), then even those distinctions between motor control and tissue extensibility become quite gray as well.

  3. John Weatherly

    Thoughtful article and exposes how the fitness industry has not even defined terms. We need a common language as you stated for communication. The NSCA calls itself “the worldwide authority on strength and conditioning” and yet we still do not have a definition for a term such as mobility? How is the NSCA “the worldwide authority on strength and conditioning?”

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  5. Flexibility is ROM for a particular joint, maybe mobility can mean that a person has flexibility throughout all areas of the body. Mobility is just flexibility in a broader sense.

  6. Jason Whaley

    I believe ‘mobility’ is redundant. We should simply use flexibility.

    Stretching, although it sounds like a passive term (maybe even ‘sissy’) to a lot of us, is what makes us more flexible and we can measure it within a test/re-test method. We call it stretching rather than flexiblization. Mobilization, though it sounds less ‘sissy’, still means a stretching action.

    We should happily use the term ‘flexibility’ in reference to the characteristic of being flexible (or its synonym, mobile)?

    However, can the term ‘mobilization’ still be useful? Surely it defines any method of mobilizing both joints and ligaments to a point of greater flexibility. For example, a Kelly Starrett, et al ‘mobilityWOD’ is one branded method for increasing flexibility that looks more functional than traditional stretching. That said, stretching is a method of mobilization that hasn’t been branded.

    As ‘mobilization’ is to CrossFit, so ‘stretching’ is to constantly varied functional movement at high intensity.

  7. Kyle

    A Definition from dictionary.com
    Defines mobility as the quality of being mobile. It then defines Mobile as the ability to move. So could we then use the term mobility as referring to the ability to move and flexibility to refer to the ROM allowed. So they can be seen as almost the same thing even interchangeable, however I would say they complement each other. Wherein mobility is literally being able to move and being flexible can allow you to move better in certain instances or scenarios. Being mobile doesn’t always require a large degree of flexibility.

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  9. Todd waldo

    Just seems like a lot of words not well spent. I would submit that flexibility is a component of mobility just like flour is a component of cake. But I’m not going to put a cup of it in my mouth to satisfy my sweet tooth just like I’m not going to just stretch to improve my mobility. I’m going to use all the ingredients. I prefer to think of mobility as the art of functional movement. I’ve seen quite a few highly flexible yoga girls at my gym who have the most broken squats I’ve ever seen and can’t run to save their lives. They’re almost TOO flexible.

    I don’t know if I’m allowed to mention it, but I’m a big fan of kstar and the mobility wod and I think he’s done a pretty good job of demonstrating how to properly MOBILIZE over the years. And yes, much of that is stretching. But not all.

    • Todd, no offense, but your personal description of what mobility means is a perfect example of the problem with this term in the fitness industry. You’ve ambiguously described mobility through analogy, and have conflated it with performance in a way that is neither necessary or quantifiable. The fitness industry has been plagued for decades by pseudoscience and marketing gimmicks. Clearly defined terms are the beginning of undermining both.

  10. Greg

    What did Kelly Starrett say? SInce he teaches the stuff weekly and is some sort of apparent God to CF’ers, he should have a pretty solid definition right?

    • I expect Kelly agrees with me. He has written on the terms “mobilization” vs. “stretching,” but has not written on the definition of mobility or flexibility to my knowledge. That being said, he seems to use the terms in a way that would indicate they are synonymous.

      • Greg

        A course titled “crossfit movement and mobility trainer course,” which states it helps coaches”…to develop effective mobility and flexibility strategies,” shouldn’t he/they/(us) be able to properly define a term which is a huge buzz word in fitness/rehab these days which he/they/(us) aim to improve through this course and books? Can we use the terms synonymously if they are used individually in the same sentence? The “and” would indicate that they are separate tasks or functions. If you can’t define something, you can measure it, and if you can’t measure it you can’t really improve it, but at the Box I attend and in the world which I work, mobility is a huge thing. The course is then (and for that matter most every CF coach’s mobility drills and recommendations) based upon helping people improve something that cannot technically be improved. Perhaps the definition of mobility could be a combo of many of the definitions listed above. Mobility as is talked about in gyms (boxes), therapy clinics is either a joint function or tends to lean towards quality of one’s movement strategies and available ROM, so perhaps the definition needs to include, and this list isn’t exhaustive clearly: Joint function (degree of joint laxity), ROM, Movement, one’s ability to control the available movement It’s clear when someone moves like crap, and then improves after practice or foam rolling/smashing/flossing/mental imagery/motor control or whatever strategy it is.

        Perhaps a Delphi study among elite CF trainers, Non CF trainers, PT’s, Chrio’s, could go a long way in helping define a term. Perhaps it could go the other way and there would be 500 completely different definitions.

      • Greg, I don’t think we need to spend time searching for some special definition for mobility. It appears to either be redundant with flexibility, speak to the diagnostics of why someone can’t achieve a full ROM (flexibility), or to conflate other well defined concepts like strength and technique. I think we need to identify that it is a word often used in a vague and unhelpful way and learn to find more specific ways to communicate.

  11. I agree with Russell that clearly defining terms must come first. I believe the confusion or misuse of the terms flexibilty and mobility arises in part because , although they are seperate and distinguishable terms, the objective measurement used to quantify each has historically been the same (range of motion). Flexibility is a term that should only be applied to those structures that are in fact able to flex, lengthen, and shorten, i.e. muscle. Mobility is a term used to quantify one aspect of the function of the neuromusculoskeletal system. It is used to describe the ability of the NMS to actively move one or more joints through a normal ROM. Mobility can be limited by lack of flexibility of soft tissue. However mobility could also be limited by the structural morphology of the joint, insufficent strength, or various other aspects of stability such as timing, coordination, etc. The latest research in neuroscience and motor control has dispelled many sacred cows in regards to flexibility and stretching. When you stretch are you actually lengthening soft tissue? Probably not. It appears what really happens is a slow, drawn out conversation with your nervous system. Through consistent and prolonged afferent input from the sensory receptors you can eventually convince your brain to reduce the neural drive or tone to the muscle. With a reduced tone the muscle can now be taken through a larger ROM.

    Let’s say someone can not abduct thier shoulder through a full range of motion. This would mean they have limited mobility in this movement, but it does not automatically mean that they do not have enough flexibility to take thier arm overhead. The rate limiting factor could be flexibility of the latissimus dorsi, pec minor, etc. However, the flexibility of the soft tissues could be normal and the rate limiting factor could be poor activation of the lower trapezius therefore reducing the necessary upward rotation of the scapula needed to complete the movement. Other limiting factors could be the structural morphology of the glenohumeral joint or strength of the rotator cuff, etc.

    Trying to say that the shoulder in the example above is not flexible when the limiting factor to movement is strength makes no sense. This is why the more global term mobility is needed. Taken to the extreme, imagine a situation where someone completely tears thier rotator cuff and can’t lift thier arm overhead. Clearly we would not say they have a lack of flexibility.

    A similiar blog could be written regarding the misuse of the terms strength and stability.

    • 1.You stated that “Flexibility is a term that should only be applied to those structures that are in fact able to flex, lengthen, and shorten, i.e. muscle.” Where is your “should” coming from? On what authority? I ask because every academic source I can find views flexibility as a measure of joint ROM, independent of what factors may be limiting that flexibility. You also note that muscles likely don’t lengthen when stretched anyway, and changes in flexibility are largely neurological adaptations. This means that any attempt to quantify “flexibility” absent the ROM of a given joint is futile.

      2. You’ve defined mobility as “…the ability of the NMS to actively move one or more joints through a normal ROM.” This is what is commonly known as “dynamic” flexibility or what I believe the soviets called “active” flexibility. These would be compared to “passive” flexibility where someone measures the ROM of my joints for me while I stay relaxed. These are well-established terms in the academic literature I have reviewed, which means if we choose your definition it is still synonymous with flexibility, just under specific circumstances (dynamic/active).

      3. I think the real problem here is that you (and many others) are conflating the diagnosis of ROM limitations with the definition of the capacity being measured. This is a very basic epistemological error. When I measure how fast someone can ride his bike from point A to point B, I call that speed. Now one person might lack speed because he is overweight. Another might lack speed simply because his bike malfunctioned. My explanation for why one of these riders is slow does not change the meaning of the term “speed.” The attachment to the term “mobility” seems to be an abandonment of the term “Flexibility” due to the perception that “flexibility” refers only to static stretching or muscle-length ROM limitations. That’s just not how the term seems to have ever been defined, and certainly not how it was measured.

  12. You say flexibility is traditionally a measurement of joint ROM. I disagree. ROM is traditionally a measurement of the available movement in a joint in a specified direction. It is the range of motion that is measured and the unit is degrees. I agree the terms need clarification and that is all I am trying to accomplish. The ‘should’ I speak of is common sense. If I am attempting to quantify the flexible nature of a tissue it should be flexible. Joint ROM in vivo is multifaceted and the flexibility of tissues is only one variable. Mobility is an umbrella term that encompasses all of the variables that contribute to the measurement of ROM. Mobility can vary depending on the pattern of movement. Example someone can lack mobility in the hip during a squat but not a lunge. Mobility can also vary actively vs. passively. I think you are very astute in your assessment of the abandonment of the term flexibility. This is exactly what is happening. The idea of ‘flexibility’ has been largely abandoned in the study of motor control and movement. At best, flexibility of soft tissue is a qualitative assessment and would more accurately be termed extensibility. You are correct in the historical use of flexibility as an all encompassing term describing ROM. Unfortunately the term flexibility was applied to ROM at a point when it was the only known variable, thus becoming synonymous. It is now a dated term with very little real scientific meaning. I think the term flexibility serves its purpose for the general public but is insufficient for clarity in cross discipline communication. If I refer someone to train with you and they lacked ROM in the hip I need to specify the source of that deficit, not speak in vague generalities. I vote to abandon the term flexibility. I believe for clarity, continuing to use the terms synonymously is counterproductive. Not that anyone cares what I think, haha. Keep up the good work. I enjoy your content.

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