The 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?
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.