What is flexibility?
Introduction
I first encountered the concept of flexibility when I began my karate training in the 1980s. In the dojo, we understood flexibility as the degree to which your joints could move fluidly through space. This was often represented by the height you could achieve in a kick, which has traditionally served as a popular, if somewhat simplistic, gauge of skill in karate. (And before anyone feels the need to argue that “high kicks don’t work in a real fight," they do work, if you’re skilled enough.) This notion of flexibility wasn’t abstract; it was understood with the utmost clarity, honed through years of practice. My instructors had relied on this definition long before my time, and it never seemed to require explanation. The alignment between flexibility and range of motion was similarly reflected in the scientific literature, where these terms have been treated synonymously for as long as research on the topic has existed. For decades, there was no debate over what we meant by “flexibility.” It was an unspoken consensus, seamlessly understood in both the dojo and the laboratory.
As the inexorable wheel of time carried us forward into the latter part of the 2000s, a fresh term began to infiltrate the dialogue of the health and fitness community: mobility. This word, wrapped in the appeal of innovation, has been extolled by its proponents as something altogether distinct from (and, intriguingly, superior to) traditional flexibility. These claims, however, have stirred a curious dissonance within the community, casting a fog over the precise meanings and implications of these concepts. Both the public and professionals alike find themselves in a haze, caught between established understandings of flexibility and the evolving rhetoric around mobility, each vying to claim a greater role in the pursuit of physical well-being.
The unsettling truth is that a significant number of advocates for mobility, in their enthusiastic zeal, fail to grasp the core distinction between mobility and flexibility. In doing so, they unwittingly promote a framework that is, in the most charitable view, educationally limited and, at its core, fundamentally misguided.
In this article, I will chart a course through the nuanced and often misunderstood realm of flexibility, seeking to clarify its core essence. Together, we will examine the distinct forms flexibility can take and identify the subtle yet significant boundaries that separate it from what we now refer to as mobility training. This article will parse the differences and show why the concept of 'mobility training,' despite its recent surge in popularity, does not, in fact, present a superior alternative to the well-founded principles of traditional flexibility training. My goal here is to recalibrate our shared understanding and anchor it in a perspective more aligned with physical reality, fostering an approach to health and training that is as evidence-based as it is effective.
What is flexibility?
The definition of ‘flexibility’ has remained surprisingly consistent throughout most of its journey from its etymological origins until today, tracing its history back to the Latin and Old French languages. At the core of ‘flexibility’ is the Latin word ‘flexibilis,’ which directly translates to “capable of being bent.” This term itself comes from ‘flexus,’ which is the past participle of ‘flectere,’ meaning “to bend.”
As the word migrated from Latin into Old French, it took the form ‘flexibilité,’ further shaping its meaning and usage within the linguistic constructs of the time. It was through this transition that the term began to embody both the physical capability of bending and the metaphorical connotations of change, adaptability, and resilience.
Upon entering the English language, ‘flexibility’ retained its foundational meanings, encapsulating the dual aspects of physical bendability and the ability to adapt to changing circumstances easily. For example, Peacock (1805) described dancers as possessing “a gentle, commanding flexibility of the joints in sinking and rising” [1]. Over 150 years later, flexibility kept its original definition of referring to the amount of movement in the joints, such as Myers (1961), who wrote on the subject of assessing joint flexibility by measuring range of motion via goniometry [2]. Fast forward another 60 years to the present day, and flexibility is still defined as a fitness capacity “that allows voluntary movements with maximum joint amplitude within physiological limits without pain or restrictions” [3].
Range of motion
‘Flexibility’ was first published in the Oxford English Dictionary in 1896 [4]. Flexibility, in its most distilled essence, embodies the ability to change, a readiness to adapt to the various demands and conditions that life may present. This interpretation is rooted in a historical lexicon dating back to 1783. In the context of human movement, the conversation naturally gravitates towards the biomechanical interpretation of flexibility. It is here, through the prism of biomechanics, a discipline that marries the immutable laws of physics with the intricate dance of human motion, that we gain a deeper understanding of flexibility and its relationship with range of motion.
Range of motion is the metric by which we quantify a joint’s flexibility. In the vocabulary of physics, ‘motion’ signifies a shift in position, a journey from point A to point B, governed by forces that sculpt the trajectory of this transition. The term 'range' can be conceptualised as a spectrum or continuum delineated by upper and lower bounds. Thus, when we articulate the phrase 'range of motion', we are essentially encapsulating the spectrum of movement a joint can traverse, from a position of anatomical neutrality to the limit of its capacity.
When applied to the human body, this confluence of motion and range transcends mere anatomical study; it becomes a metaphor for the human condition, illustrating the physical capacity for change, adaptation, and resilience. In this light, range of motion is a metric of physical flexibility and emblematic of the broader human capacity to navigate the fluctuations of existence.
However, despite their frequent synonymy, we must acknowledge a subtle distinction between flexibility and range of motion. While both terms indeed refer to the degree of positional change a joint can achieve, they illuminate different facets of this change. Flexibility, with its broader connotations, encompasses a physical readiness to adapt, whereas range of motion provides a quantifiable measure of this adaptability within the specific context of biomechanical limits. We encounter a similar scenario in strength training, in which the load moved is an expression of a muscle’s capacity to produce force.
As we dissect the subtleties of flexibility further, we encounter a spectrum of types, each reflecting distinct conditions and capacities for movement. This categorisation invites a deeper exploration of the conditions under which flexibility manifests, reinforcing the idea that flexibility, in both the physical and metaphorical sense, is a multifaceted concept with varying degrees of expression and significance.
The four types of flexibility
Flexibility consists of two principal conditions that delineate its essence: firstly, the characteristics inherent in the movement of joints, and secondly, the nature of muscle activation. In the former, we confront a common misconception surrounding the concept of 'acceleration'. Contrary to the lay understanding that equates acceleration exclusively with the notion of speeding up, in physics (from which biomechanics, our discipline of interest, derives its foundational principles) acceleration encompasses any change in speed. This understanding is crucial when considering human movement, which inherently involves a transition from one state of stillness to another, punctuated by intervals of speeding up and slowing down.
Take, for instance, the act of performing a squat. This movement begins from a standing posture, from which we initiate a descent, accelerating as we lower ourselves. This acceleration is not sustained indefinitely; rather, as we approach the lowest point of the squat, we decelerate, coming to a complete halt before reversing the motion to ascend, once again experiencing a cycle of acceleration and deceleration until we return to our original stance. This process exemplifies the ubiquity of acceleration in all forms of human movement, challenging us to reconsider our preconceptions and appreciate the complex dynamics at play.
Turning our attention to the lexicon of biomechanics, we encounter the terms 'dynamic' and 'static', which pertain to the presence or absence of acceleration, respectively. ‘Static’ is synonymous with a state of immobility, and 'dynamic' is synonymous with movement. In discussing flexibility, we employ these terms to describe whether a joint is engaged in movement or remains stationary during a particular expression of flexibility.
The dichotomy of 'active' versus 'passive' states offers another layer of complexity on the spectrum of muscle activation. 'Active' muscle contraction is characterised by the deliberate use of chemical energy to facilitate voluntary movement, whereas 'passive' refers to a muscle in a state of relaxation devoid of active contraction. An important point to remember is that the states of joint movement and muscle activation are inextricably linked, each influencing and informing the other. In other words, a joint is always either static or dynamic; a muscle is always either active or passive.
This interdependence gives rise to four distinct categories of flexibility: dynamic active, dynamic passive, static active, and static passive. Every movement, stretch, or mobilisation we engage in can be categorised within this framework, illustrating the vast possibilities for movement and expression through our bodies. It is possible for a movement or posture to transition between these states.
What flexibility is not
Advocates of mobility training frequently refer to flexibility as 'passive range of motion', a term that denotes the passive lengthening of tissues. While not entirely devoid of merit, this characterisation simplifies the concept to a point where it obscures better understandings of human physiology and flexibility. We must recognise that flexibility encompasses far more than a passive state; it also includes dynamic active flexibility cultivated through dynamic stretches. This distinction is underpinned by thousands of studies dedicated to exploring how dynamic stretches enhance flexibility, indicating a breadth of understanding that cannot be encapsulated by the notion of flexibility as merely passive.
When confronted with the assertion that flexibility is solely passive range of motion, one must question the exclusion of dynamic flexibility from this definition. This inquiry is not pedantic but foundational, for it challenges a misconception that undermines the intricacy of human movement and the adaptability of our tissues. The active lengthening of tissues, an aspect integral to dynamic flexibility, illustrates that our bodies are capable of both passive and active adaptations in response to physical demands.
Moreover, the term 'flexibility' itself, often used colloquially to describe the capacity for movement, is not the precise term favoured in the scientific literature when referring to the lengthening of tissues. Instead, 'extensibility' is employed to denote the potential for a tissue to stretch or lengthen in response to an external force. This distinction speaks to a broader confusion in the dialogue surrounding physical conditioning, a conflation of terms that, while related, signify different aspects of our physical capabilities. Extensibility is a component of flexibility, but to conflate the two is to overlook the nuances that distinguish passive capacity for movement from the active engagement and development of our physical potential.
What is mobility?
Any conversation on the definition of flexibility must confront the distinction between flexibility and mobility, a distinction that, while subtle, reveals profound insights into the mechanics of the human body and the principles of physical training. Flexibility, as we've come to understand, denotes the capacity of a joint or sequence of joints to move through a given range of motion. This characteristic manifests in four distinct types, each defined by the specific interplay between the joint(s) involved and the muscle(s) acting upon them. Yet, when we turn our attention to mobility, we encounter a term mired in ambiguity, its significance frequently distorted by varied interpretations within the health and fitness domain. These interpretations, regrettably, often stray from the rigour of scientific discourse.
The prevalent definition of mobility, particularly within the sphere of health and fitness, hinges on the concept of active range of motion, i.e., the extent to which we can autonomously articulate a joint. Proponents of this perspective, who frequently have a commercial interest in promulgating mobility training programmes, juxtapose mobility with flexibility, characterising the latter as passive range of motion, or the degree to which an external force can manoeuvre a joint. However, this dichotomy reveals an oversimplification that fails to accommodate the complexities of dynamic versus static flexibility, nor does it consider the velocity of movement, an oversight that undermines the validity of this model.
The essence of this debate extends beyond semantic distinctions; it touches upon the fundamental mechanisms by which our bodies adapt to the stimuli they are subjected to. Indeed, the human body is equipped with both a dynamic stretch reflex and a static stretch reflex, evolutionary adaptations fine-tuned to respond to the specific nature of the stimuli they encounter [5]. Alterations in velocity trigger the dynamic stretch reflex, while the static stretch reflex activates in response to a muscle being maintained at a constant length. This delineation underscores the principle that our bodies do not merely react to external forces, but rather, they adapt in a manner that is exquisitely attuned to the nature of the stimulus.
Thus, when we contemplate the distinction between flexibility and mobility, we are, in essence, exploring the interplay between passive and active movement, between the potential for motion and the execution of that motion under the directive of our own volition. This exploration holds practical implications for how we approach physical training, underscoring the necessity of tailoring our training regimes to reflect the specific adaptations we seek to engender within our bodies.
In a ironic and revealing manner, the common jargon surrounding 'mobility' (typically invoked to denote the active range of motion) misaligns dramatically with its scholarly definitions. Delving into the biomechanical literature reveals a different understanding, wherein 'mobility' is construed as the degrees of freedom afforded by a joint or a chain of joints [6]. Here, a 'degree of freedom' represents a potential vector of movement for a joint, encapsulating the totality of directions it may traverse. Notably, these degrees of freedom are immutable, intrinsic properties of the joint's anatomical and physiological constitution, not amenable to training or enhancement in the traditional sense.
This brings us to a peculiar juncture where educators and proponents of so-called 'mobility training' claim to augment these degrees of freedom through their methodologies, a proposition that, under scrutiny, reveals a fundamental misunderstanding. The truth, often obscured by semantics, is that what can indeed be expanded is the amplitude of movement within these predefined vectors: the range of motion, to be precise. It's a significant distinction, highlighting a conflation of terms wherein 'mobility training', as it is popularly conceptualised, veers closer to what is traditionally understood as flexibility training.
The irony deepens when one considers the disdain with which these mobility coaches often regard flexibility training, positioning it as somehow lesser or inferior, despite their practices essentially championing the same end. This discordance between terminology and practice sows confusion and underscores a broader issue within the field: a lack of clarity and consensus on fundamental definitions. By navigating these semantic intricacies critically, we stand to gain a more accurate and productive understanding of human movement, unencumbered by the constraints of misapplied labels.
In the discipline of motor control, scholars and practitioners alike converge on a singular definition of mobility: it is the capacity to move oneself from one location to another independently and safely [7]. This definition unfurls to encompass many daily activities that many of us take for granted, such as getting out of bed, bathing, dressing, and navigating stairs. While seemingly mundane, each activity constitutes a critical facet of one's ability to engage meaningfully with the world. Within this context, there exists an array of devices and implements, collectively referred to as mobility aids, designed to increase independence in the presence of disease or disability (wheelchairs, walking canes, etc).
Even in research papers where the term 'mobility' is employed to denote the concept of range of motion, a consistent observation emerges. With remarkable uniformity, authors delineate both active and passive range of motion under the umbrella term of mobility [8]. Put simply, the term ‘mobility’ is used as a surrogate for 'flexibility.’ In this context, mobility and flexibility are exactly the same thing, albeit with a much more limited conceptual model than the four types of flexibility.
Summary
Flexibility is defined as the capacity of a joint or sequence of joints to traverse through a range of motion. This idea, while seemingly straightforward, bifurcates into four distinct categories predicated upon the actions of the joints and muscles involved: dynamic active, dynamic passive, static active, and static passive. Each category elucidates a different aspect of how our bodies interact with the forces of motion and resistance.
Modern-day conversations around flexibility often see mobility training posited as a superior approach. However, this assertion emerges from a misunderstanding of what flexibility entails and a limited reading of the scientific literature on the subject. To argue that mobility training supplants flexibility training is to ignore the interplay between muscle and joint function that flexibility encompasses. Most purportedly advanced mobility regimens are, upon closer examination, merely iterations of traditional flexibility training, albeit cloaked in a veneer of novelty and predicated upon narrowed conceptual frameworks.
This rebranding of flexibility training as mobility training muddles the discourse and detracts from a holistic understanding of human biomechanics. It fails to acknowledge the depth of research and knowledge that underpins flexibility training, reducing it to oversimplified models that do not capture the full spectrum of human movement. To engage with this topic thoroughly, one must look beyond superficial distinctions and appreciate that flexibility is essential to our physical well-being.
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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.