What is flexibility?
This post first appeared on my original 'Splits and High Kicks Blog' back in 2010. The most recent update was February 2024.
Introduction
I first got serious about flexibility when I started karate back in the 1980s. In the dojo, we didn’t overcomplicate things. Flexibility meant how far your joints could move and, more importantly, how high you could kick. That was the standard. If you could snap a roundhouse to someone’s head with ease, you were flexible. Of course, some people love to argue and claim that high kicks don't work in a street fight, but they work just fine if you know what you're doing. Flexibility wasn't some vague or theoretical concept. It was something you felt, something you developed through countless hours of stretching, kicking, and training. My instructors never had to sit us down to explain at length what flexibility was; they didn't have to. We just understood the concept. The teachers who came before me trained the same way, and it worked, so we didn't question it. Science backed it up, too. Almost every study on flexibility and range of motion treated them as the same thing. For decades, everyone from martial artists to researchers knew exactly what "flexibility" meant.
Sometime in the late 2000s, a new buzzword started making the rounds in the health and fitness world: mobility. It showed up everywhere, dressed up like some revolutionary concept, and before long, people were talking about it like it was the next big thing. They claimed it was something totally different from plain old flexibility (and, if you listened to the hype, way better). But the problem is, with all this talk about how mobility was supposed to be something special, a lot of people, including trainers, athletes, and regular gym-goers, started getting confused. Suddenly, flexibility wasn’t enough. Now, you had to be have "mobility," whatever that meant. And just like that, the conversation turned into a mess. People weren’t sure what was what anymore. Was mobility just flexibility with a new name? Was it something better? Or was it just another fitness trend trying to sound smarter than it really was? Either way, both the experts and the everyday exercisers found themselves stuck in the middle, trying to figure out whether they should stick with what they knew or jump on the new bandwagon.
The truth, whether people like it or not, is that a lot of so-called mobility advocates don’t actually understand the difference between mobility and flexibility. They get caught up in their own hype, pushing ideas that, at best, barely scratch the surface of real education and, at worst, send people down the wrong path entirely.
People love to talk about flexibility but they rarely understand it. So, in this article, we're going to break down what it really is and how it works. More importantly, we're going to establish why it's not inferior to mobility training. A lot of people think mobility is some kind of upgrade, like a next-level version of stretching, but that's not the case. Flexibility isn't outdated or obsolete. We'll take a look at the actual differences between flexibility and mobility, and figure out where one ends and the other begins. Because if we're going to train smarter, we need to base our approach on how the body actually works, not just what's trending in the fitness world. By the time we're done, you'll have a much clearer picture of what flexibility really means and why it still matters.
What is flexibility?
Flexibility has always been about how far you can move. It’s a concept that’s stayed rock-solid from its early roots in Latin and Old French to how we define it today. The Latin word flexibilis means exactly what it sounds like: “capable of being bent.” That word comes straight from flexus, which is the past participle of flectere, meaning “to bend.” It's a straightforward idea that has stood the test of time. When the word made its way into Old French, it evolved into flexibilité. That’s when people started realising it wasn’t just about physical movement, but that it was about adaptability, resilience, and the ability to change when needed. The French took a good concept and stretched it further (pun intended), making it a word that belonged anyone who knew how to change to roll with the punches.
Once flexibility landed in English, its meaning held onto both physical bendability and the ability to adjust to whatever came your way. In 1805, Peacock described dancers as having “a gentle, commanding flexibility of the joints in sinking and rising” [1]. That makes sense. If you’re moving, you need flexibility, whether you’re throwing a high roundhouse kick or dancing across a stage. Then, about 150 years later, Myers (1961) was still talking about it the same way. He focused on joint movement, measuring flexibility with goniometry to get precise readings on range of motion [2]. Fast forward another six decades, and the definition still holds. Today, it’s described as a fitness capacity “that allows voluntary movements with maximum joint amplitude within physiological limits without pain or restrictions” [3]. In other words, if you can move without feeling like a rusty hinge, you’ve got flexibility.
Range of motion
Flexibility first made its way into the Oxford English Dictionary in 1896 [4]. At its core, flexibility is about adapting. It’s about changing to meet whatever challenge life (or movement) throws at you. The word itself has roots going all the way back to 1783, but the idea has been around since the first person had to bend, twist, or dodge something coming their way. Now, when we talk about flexibility in human movement, the conversation naturally shifts to biomechanics. That’s where things start getting interesting. Biomechanics is where physics meets human motion; it’s how we understand why and how we move the way we do. And in the world of movement science, flexibility is related to range of motion.
Range of motion is how we measure flexibility. In physics, motion means a change in position, or a shift from point A to point B. But movement isn’t just about getting from here to there. It’s about how much space you can cover while doing it. Range gives us the boundaries of that movement. So when we talk about range of motion, we’re talking about how far a joint can go from a neutral starting point to its maximum limit. But range of motion is about people as much as it's about joints. It’s a perfect metaphor for life itself. Just like a joint moves within its limits, we move within the limits of our own circumstances. Our ability to change, to adjust, and to adapt is what range of motion really represents. It’s proof of our ability to physically navigate life’s ups and downs.
Now, here’s where people get confused: flexibility and range of motion aren’t the same thing. They’re close, but they’re not identical. Flexibility is about being ready to move. Range of motion is about how far you can actually move. It’s kind of like strength training: lifting a heavier weight shows how strong you are, but strength itself is a broader concept. The same goes for flexibility. It’s a readiness to move or to adapt, while range of motion is the specific, measurable aspect of that movement. If you dig deeper, you’ll see that flexibility isn’t just one thing. It’s a spectrum. Different conditions beget different movements, which leads to different types of flexibility all coming into play. And just like in life, some people have more, some people have less, but everyone can improve. Whether we’re talking about stretching, training, or just rolling with the punches, flexibility is about adjusting to the moment and making the most of what you’ve got.
The tetradic model of flexibility
Flexibility boils down to two things: how your joints move and how your muscles fire. But a lot of people get mixed up when it comes to movement, especially when they hear the word “acceleration.” Most folks think acceleration just means “speeding up,” but that’s not the whole picture. In physics, and in biomechanics (which is just physics applied to the human body), acceleration means any change in speed. That means speeding up, slowing down, or stopping. And since every movement we make starts from a standstill, speeds up, and eventually slows down, acceleration is happening all the time.
Think about a simple squat. You start standing still. Then, as you lower yourself, you accelerate downward. But you don’t just keep going down forever: you slow down before you hit the bottom, coming to a brief stop before pushing back up. On the way up, you accelerate again, gaining speed, until you reach the top and slow down back to your original stance. All movement consists of acceleration and deceleration. Most people don’t think about it that way, but once you do, it changes how you see every movement you make.
Now, in biomechanics, you’ll hear the words dynamic and static all the time. They’re really just fancy ways of saying "moving" or "not moving," respectively. A joint is either dynamic (it’s moving) or static (it’s holding still). And when we talk about flexibility, that’s the first distinction we need to make: are we stretching while moving or while staying still?
But that’s only half the story. The next thing to look at is whether the muscles are active or passive. Active means the muscles are contracting, doing work, and using energy. Passive means they’re relaxed, just being stretched without actually firing. Every stretch, every movement, and every position we do falls into one of these four categories:
- Dynamic Active: Moving, and your muscles are doing the work.
- Dynamic Passive: Moving, but your muscles aren’t actively helping; something else (like gravity or an external force) is guiding the movement.
- Static Active: Holding still, but your muscles are engaged to keep you there.
- Static Passive: Holding still, but your muscles are relaxed, with an external force keeping you in position.
Every stretch or movement fits somewhere in this framework. And just because something starts in one category doesn’t mean it stays there. Movements transition from active to passive, from dynamic to static, and back again. Understanding this gives you a deeper awareness of your own body and how you move.
I call this framework the Tetradic Model of Flexibility. Tetradic just means it’s built around four parts. It's simple, but it covers every kind of flexibility, and once you see movement through this lens, you start to recognise patterns in how your body works. And that’s when training really gets interesting.
What flexibility is not
The mobility folks talk about flexibility like it's just passive range of motion, as if stretching only involves sitting there while your tissues get stretched out by an external force. Sure, there’s some truth to that, but calling it just passive range is like saying fighting is just about throwing punches. It’s only part of the story, and if you stop there, you’re missing the whole point of what flexibility really is. Flexibility goes way beyond how far you can stretch when someone pushes you into position. It’s about how well you move, how well you control that range, and how much of it you can actually use. That’s where dynamic active flexibility comes in. You don’t just get functional flexibility by sitting in a stretch; you get it by moving, kicking, and using those ranges over and over again. Thousands of studies back this up, showing that dynamic active stretching is how your body uses the flexibility you build in static passive and static active exercises.
Now, when people tell you flexibility is only passive range of motion, stop and think: where’s the dynamic part? Why cut that out? That’s not just some tiny mistake; it’s a big one. Ignoring dynamic flexibility means ignoring how the body actually works. Your tissues lengthen and strengthen when you move. It’s the difference between having flexibility and using it. If you’re in a fight and your leg can barely lift past your waist, what good is it if you can do the splits on the floor? What I'm talking about here is the practical application of flexibility during movement.
And here’s something else: even the word flexibility gets thrown around too loosely. People think it just means "how far your muscles can lengthen," but in actual science, the correct term for that is extensibility, which is how much a tissue can lengthen when an external force is applied. It’s a small but important difference. Extensibility is part of flexibility, but, like range of motion, it’s not the same thing. Confusing the two is like saying punching and blocking are the same just because they both involve your arms.
Flexibility isn’t just about stretching, and it sure isn’t just passive. It can also be dynamic, it can also be active, and it’s something you train, not just something you have. Because at the end of the day, if you can’t use it, you don’t really have it.
What is mobility, then?
When people talk about flexibility, they always seem to mix it up with mobility. It sounds like a small difference, but trust me, it’s not. Flexibility is about how far a joint, or a group of joints, can move through a given range of motion. As we have seen, there are four types of flexibility, each working a little differently depending on how the muscles and joints interact. But when we start talking about mobility, things get messy. Everyone has their own take on what it means, and a lot of times, that take isn’t based on science, it’s just what some trainer or coach thinks sounds good.
In the fitness world, mobility is usually described as "active range of motion," or how far you can move a joint on your own. The folks pushing mobility training often make this big contrast between flexibility and mobility, claiming flexibility is just "passive range of motion," or how far an external force can stretch you. Sounds reasonable, right? Not really. This black-and-white way of thinking ignores the difference between static and dynamic flexibility. And it completely overlooks how movement speed affects flexibility. If you leave that out, you’re missing a huge part of the picture.
This issue goes beyond a simple debate over the meaning of words and extends into contradicting what we know about how the body works. The human responds to stretching in very specific ways [5]. We’ve got two built-in stretch reflexes: one for quick movements (the dynamic stretch reflex) and another for slower, held stretches (the static stretch reflex). When movement speed changes, the body switches between these reflexes automatically. So, when we talk about flexibility versus mobility, what we’re really discussing is the difference between what your body can do (passive flexibility) and what it will do when you're in control (active flexibility).
Now, here’s where it gets humorous. The word mobility is used all the time in fitness circles to mean active range of motion. But if you look at biomechanics research, it actually means something completely different. There, mobility refers to how many ways a joint could move, not how far it can go [6]. That’s called degrees of freedom, and it basically refers to how many different directions a joint can move in. And here’s the kicker: you can’t increase the degrees of freedom of a joint. That’s set by your anatomy. No amount of mobility training will magically add new movement possibilities to your joints.
But go ahead, listen to these so-called mobility experts, and they’ll tell you they’re "increasing joint mobility" with their special training methods. What they’re actually doing is increasing range of motion within existing degrees of freedom, which is, surprise, flexibility training. And yet, many of these same mobility coaches treat flexibility training like it’s outdated or useless. Oh, the irony. They claim mobility training is superior, but in reality, they’re just giving flexibility training a new name. This kind of linguistic confusion makes it harder for people to understand what’s really happening in their bodies. If we cut through the noise and get to the truth, we’ll be in a much better position to train effectively and improve movement without getting hung up on marketing buzzwords.
In motor control studies, mobility has an entirely different definition. It doesn’t mean range of motion at all. Instead, it refers to a person’s ability to move themselves from one place to another [7]. Walking, climbing stairs, getting out of bed, all of that falls under mobility. And in this context, we use the word to describe assistive devices like wheelchairs or canes that help people stay independent when their mobility is compromised.
Even in research papers that use mobility to talk about range of motion, there’s still a consistent pattern. They always include both active and passive range of motion under the same term [8]. That means, in those studies, mobility and flexibility are actually being used interchangeably. It’s just a more limited way of talking about flexibility, and it's one that doesn’t even recognise all four types of flexibility.
So, what’s the takeaway? If you want to improve your range of motion, you’re training flexibility, whether you call it that or not. And if you’re trying to move better, it’s about developing strength and control over the flexibility you already have. (This is called flexibility-strength rather than mobility.) The more you understand this, the better your training will be.
Summary
Flexibility is simple: it’s how much a joint (or a series of joints) can move through a range of motion. When we break it down, there are four types: dynamic active, dynamic passive, static active, and static passive. Each one plays a different role in how your body handles movement, resistance, and control.
These days, you hear a lot of talk about mobility training as if it’s some kind of upgrade to flexibility training. But that idea comes from a misunderstanding of what flexibility actually is. People like to throw around new terms to sound advanced, but at the end of the day, mobility training isn’t replacing flexibility. It’s just a different way of looking at the same thing. A lot of so-called "new" mobility programs are just traditional flexibility exercises with a fresh coat of paint. The principles haven’t changed, but the marketing has.
The problem with this rebranding is that it confuses people. It takes something well-researched, tested, and proven, and repackages it in a way that ignores the full picture of how the body works. When you strip flexibility down to a narrow definition, you miss out on the bigger picture: that flexibility is a fundamental part of movement, performance, and injury prevention. If you really want to understand it, you have to go beyond the buzzwords and look at the science. Flexibility is not outdated. It’s essential.
References
1. Peacock, F. (1805) ‘Sketches relative to the history and theory, but more especially to the practice of dancing.’ J. Chalmers and Company.
2. Myers, H. (1961) ‘Range of motion and flexibility.’ Physical Therapy, volume 41, number 3, pages 177-172.
3. Niaradi, F. et al. (2024) ‘Effect of eutonia, Holistic gymnastics and Pilates on hamstring flexibility and back pain in pre-adolescent girls: Randomised clinical trial.’ Journal of Bodywork and Movement Therapies, epub ahead of print.
4. Flexibility, N. meanings, etymology and more | oxford english dictionary. Available at: https://www.oed.com/dictionary/flexibility_n
5. Levin, M. & Feldman, A. (1994) ‘The role of stretch reflex regulation in normal and impaired motor control.’ Brain Research, volume 657, number 1-2, pages 23-30.
6. Li, Q. et al. (2016) ‘Mobility analysis of limited degrees of freedom parallel mechanisms in the framework of geometric algebra.’ Journal of Mechanisms and Robotics, volume 8, number 4, article 041005.
7. Shumway-Cook, A. et al. (2023) Motor control: Translating research into clinical practice. Philadelphia: Wolters Kluwer.
8. James, B. & Parker, A. (1989) ‘Active and passive mobility of lower limb joints in elderly men and women.’ American Journal of Physical Medicine and Rehabilitation, volume 68, number 4, pages 162-167.