What is Flexibility?

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What is Flexibility?

Abstract

This article looks at what physical flexibility is and how it should be defined and understood. Flexibility is a motor ability of the body. It lets a joint, or several joints working together, move freely through a range of motion without causing discomfort or risking injury. Flexibility does not come from one source. It depends on the structure and arrangement of tissues that bend and slide, and on the control of the nervous system, which sets the limits of movement the body will allow.

Medical disclaimer

This content is for education purposes only. It does not replace medical advice. Do not start or change exercise or rehabilitation programmes without consulting a qualified healthcare provider.

Last updated: 4th October 2025.

Change log: Article reformatted.

Key points (TL;DR)

Flexibility is joint-specific range of motion shaped by tissue mechanics and neural stretch tolerance.
Flexibility is often divided into active and passive sub-types, but this view is limited and does not account for other conditions.
Flexibility is more accurately divided into four sub-types: static passive, static active, dynamic passive and dynamic active.

Table of contents

1. Introduction

Many people devoted to health and fitness search for a clear and practical answer to a question that appears simple: “What is physical flexibility?” We hear it in clinics, gyms and on sports fields, but the definition shifts. Some say it is the range of motion of a joint. Others say it is the ability of tissues to lengthen. Still others describe it as the mental and physical capacity to withstand the discomfort of a stretch. This article sets out to bring clarity to the term, to separate it from related ideas and to show how it can be measured and developed through training.

The answer I give is built on precise definitions found in physics, sports medicine and public health. I look at the peer-reviewed research that explains the mechanisms behind flexibility and the methods used to improve it. Along the way, I separate established facts from claims that are still uncertain or not yet supported by solid evidence. My goal is to form a definition that anyone, regardless of their background, can grasp, restate in their own words and apply in their training.

2. Definitions and scope

Flexibility is the ability of a joint to move through the full span of its anatomical motion. The American College of Sports Medicine states that flexibility is the ability to move a joint through its complete range. ^[1] The National Academies define it as a fundamental property of tissues, a hidden quality that sets the safe limits of joint motion, establishing the delicate balance between movements that can be performed and those that must be avoided. ^[2]

“Flexibility” is commonly used in place of “range of motion.” For practical purposes they mean the same. To explain range of motion is to show why it is the main way to measure joint flexibility. Range of motion is a working measure of movement. It shows all the positions a joint can take. It depends on bone structure, on the stretch properties of connective tissue and on the action of muscles. In both practice and research, two forms are noted. Active range of motion comes from the effort of the muscles that move the joint. Passive range of motion comes from an outside force moving the joint. Both are part of the same rules of joint mechanics. To make the terms exact across medicine, therapy and sport, researchers use coordinate systems set out by Grood and Suntay. These systems let them describe joint rotation and translation in three dimensions with accuracy. This gives a common language for the study of human movement. ^[3,4]

Etymologically, “range” carries the sense of extension and breadth. It comes from Old French, meaning “to be set in a row,” and goes back through Germanic roots dealing with bending and turning. “Motion,” on the other hand, comes from the Latin motio, meaning “a moving,” itself from movere, “to move.” Put together, the phrase gains a clear and exact meaning. Physicists use it in kinematics: motion is a change of position, range is the span of states this change can take. In biomechanics the phrase has anatomical precision. Here, motion is tied to the axes of joints, and range is tied to the limits and freedoms that define their scope of movement.

Flexibility is the ability of the body to move through a wide range of positions, but it is also the ability of the joints to change their position according to physical demands. Coaches and therapists often divide it into “active” and “passive” ranges of motion. This division is useful, but it is incomplete and oversimplifies the matter. A more precise classification recognises four types: static passive, static active, dynamic passive and dynamic active. This scheme distinguishes whether the joint is measured or trained in stillness or in motion, and whether the movement comes from outside forces or from the individual’s own muscular effort.

3. Background and theory

Flexibility depends on two main mechanisms. The first comes from the mechanical properties of tissues: joint capsules, ligaments, fascia, and muscle–tendon units. These structures resist elongation and in doing so protect the joint’s integrity. Because they are viscoelastic, their resistance changes with time. When held under stretch, their resisting force gradually diminishes. This process is called stress relaxation. Earlier studies linked limited motion to higher passive stiffness and reduced stretch tolerance in the hamstrings. ^[5] Later research indicated that careful, repeated static passive stretching can increase fascicle length. This points to a modest but real structural change. The influence of stretching on tendon viscoelasticity, however, is still not clear, with studies showing different results. ^[6,7]

The second main mechanism is neural regulation and stretch tolerance. What stops movement at its limit is not the tissue itself, but the feeling of discomfort during extension and the reflexes it provokes. The growing body of research shows that gains in range after stretching come not so much from big structural changes in the body, but from learning to accept the feeling of stretch. ^[8] Recent reviews confirm that tolerance is the key factor, with structural changes also occurring, but they are smaller and vary in form. ^[9]

4. Evidence Synthesis

The distinction between static and dynamic concerns how tissues behave when held in one place versus how they perform in motion. In stillness, tissues show their time-dependent, viscoelastic properties. In movement, they show another behaviour, one governed by the constant interaction of elasticity and resistance. The division between passive and active is equally important. Passive means the limits set by the tissues that surround a joint. Active means the effect of the nervous system and the force of muscles working to move through a range. Reviews of the subject and the general agreement among specialists support these distinctions. Each shows a separate level of how flexibility manifests. In practice, we can take static positions passively, with help from an external force, or actively, by our own effort. Dynamic movement covers both controlled, deliberate actions and faster, stronger motions that show readiness for sudden effort. ^[10]

Static passive flexibility shows how pliable the muscle–tendon system is and how the body adjusts to the feeling of stretch. Reviews of the mechanisms explain that the quick but short-lived increase in range of motion usually comes from a change in how much stretch discomfort the body accepts, although length changes in the tissues can also play a part. ^[11] When this training continues over longer periods, passive stiffness drops and lasting improvements in range of motion appear. Large meta-analyses of stretching programmes carried out over weeks confirm these results, and the evidence makes the point with great clarity. ^[12,13]

Static active flexibility means producing torque at the end of your range of motion. Research showing that resistance training can improve flexibility as much as traditional stretching proves that strength and motor control set the limits of movement. ^[14] Performance tests show that short static holds do not reduce strength or speed, but long holds (≥ 60s) lower both. This shows the fine link between neural drive and the control of movement at its limit. ^[15,16]

Dynamic active flexibility is the range of controlled motion in which one set of muscles contracts while the opposing set lengthens. It is also defined as flexibility displayed at speed. Each movement you can imagine and then perform is an example of this ability. Dynamic active stretching is the main means of developing it. The results are usually small but useful: sometimes a slight gain in performance, sometimes no noticeable effect and on rare occasions a decrease. For this reason, dynamic active stretching fits well into warm-up routines. ^[17,18]

Dynamic passive flexibility is movement produced by an external force, not by your muscles. The joints travel through their range without voluntary contraction. Continuous passive motion (CPM) machines apply this principle. They move the body as if guiding it through a controlled dialogue. Meta-analyses describe CPM with accuracy, recognising it as dynamic passive range of motion in practice. CPM does not give better long-term results than active therapy. But it shows patterns in tissues and reveals clinical effects that active movement alone does not bring out. ^[19,20]

Research agrees that all four domains of flexibility are important, because each shows different limits and their results. Static passive shows how stiff tissues are and how discomfort the nervous system can tolerate. Static active adds strength to this picture and shows the fine control of muscles and nerves. Dynamic active connects to the demands of real performance, where ability is tested by steady and purposeful movement. Dynamic passive shows how tissues and joints act when moved by outside forces, giving a view of their reactions when there is no intent to move. Recent consensus builds on this, bringing more order to the definitions of stretching and making these distinctions stronger, fixing their role in both practice and study. ^[21]

5. Practical Implications

When flexibility is divided into four distinct categories, we gain a precise language for identifying what is measured, what is trained and what results can be expected. With this language, uncertainty is removed, clarity is established and training is directed by clear intention.

Static passive flexibility shows the current state of your tissues and how they handle stress under stretch. It tells you when and how to train it, either with short, concentrated holds or with long, mild loading. Research reviews prove that several weeks of static passive stretching provide steady gains in range of motion. They also show that the method used, (continuous angle or continuous force) and the amount applied, both make a clear difference. Progress is determined by how often you encounter the limit of passive tension. You can get there by learning to live with the discomfort of the stretch or by changing how much tension the muscles and other tissues create during a static passive stretch. ^[13]

The changes in structure are not large, but they do exist. Static passive stretching can bring small increases in fascicle length, and these increases correspond with the changes in how you feel and interpret the stretch. ^[6] What we know with great certainty is that static passive stretching increases static passive range of motion over time. When dynamic and active methods of training fail to bring significant gains in flexibility, the most likely reason is their inability to change the limit set by static passive range of motion. That limit remains the ultimate boundary that controls all other kinds of flexibility.

Static passive range of motion is the real measure of flexibility. Other types of flexibility only show how much of this range you can use in specific conditions. Dynamic active flexibility is simply that part of your static passive range of motion you can display when moving at speed. Static active flexibility shows how much of your static passive range of motion you can demonstrate while the muscles are working under tension.

From my experience, though not yet fully confirmed in peer-reviewed research, I have observed a pattern worth attention. People who build flexibility only through active methods, such as isometric contractions or resistance training over full ranges of motion, eventually struggle to display static passive flexibility. With time, they need more warm-up sets, more preparatory stretches or longer stays in a stretch to reach the same range of motion they once reached quickly. The cause, in my opinion, is greater stiffness of the connective tissues and/or reduced tolerance for passive stretch, even though these athletes can generate much higher active tension in the lengthened muscles.

The nervous system tells the difference between active and passive tension in stretching by matching signals from the periphery with copies of motor commands. Context at spinal and supraspinal levels moderates this process. In the periphery, muscle spindles provide information about muscle length and velocity. Their sensitivity depends on γ fusimotor drive, which rises during voluntary exertion. Golgi tendon organs report tension in the tendon with notable accuracy under both active and passive load. Central circuits compare these signals with the efference copy, which is a duplicate of the brain’s command to the body. This copy lets the nervous system forecast the expected result. From this comparison, separate evaluations of “effort” and “force” emerge. As a result, identical joint torques may feel fundamentally different depending on whether they are produced voluntarily or imposed from the outside.

Muscle spindles provide feedback that is sensitive to both muscle length and the speed of change, carried through group Ia and II afferents. During active movement, the γ drive activates intrafusal fibres together with the main muscle fibres, which keeps the spindles taut. This ensures spindle signals focus on the aspects of motion most important to the movement being performed. Modern research in microneurography, supported by computational models, shows that human spindles work as controllable signal processors, influenced by descending commands and the demands of the task, and not as simple, passive gauges of length. When a muscle undergoes passive stretching, γ drive remains low. In such cases, the spindles mainly signal the imposed lengthening and its speed, which raises the gain of the stretch reflex, unless presynaptic inhibition reduces Ia transmission.

Golgi tendon organs, working through group Ib afferents, report on musculotendinous tension with an almost linear response, whether the tension comes from active contraction or passive stretch. Their signals give the nervous system feedback about load, which can either inhibit or facilitate motoneurone activity by way of spinal interneurons, with the result depending on the behavioural state at that moment. Human studies using electrical stimulation of tendons show that inhibitory reflexes triggered at the tendon grow stronger after substantial loading, which matches the adaptable character of Ib gating. Because Ib afferents carry information about tension regardless of its source, the central nervous system compares this input with the efference copy, making it possible to distinguish between tension produced by voluntary effort and that caused by an external load.

At the spinal level, static passive stretching quickly reduces the size of both the H-reflex and the tendon-tap reflex. This happens within minutes and shows that motoneurone excitability decreases while presynaptic inhibition at the Ia terminals increases. When such passive stretches are held for a long time, estimates of persistent inward currents in motoneurones also decrease. With these currents reduced, the amplification of synaptic drive is lessened and the responses produced by stretch become weaker. These changes take place without reliable alterations in corticospinal excitability. This means the immediate control of reflex gain during passive lengthening is determined by spinal mechanisms, not cortical ones.

In supraspinal regions, the ability to withstand the limits of stretch comes from the systems that govern pain. Tolerance depends on how well the body’s own inhibitory mechanisms work. Descending control from central structures sets the limit of how far a joint can be moved passively before discomfort stops further movement. The periaqueductal grey and the rostral ventromedial medulla act both ways. They can block or amplify pain signals. They regulate the input from group III and IV afferents in muscle and connective tissue. These afferents react to mechanical strain and to the chemical conditions present at extreme lengths. The balance of these systems fixes the exact point where passive tension is recognised as a threat.

But why does the range of motion at extreme muscle lengths so often decline when we rely only on active flexibility methods while avoiding static passive stretching, even if the position was originally achieved through active effort? Several mechanisms may explain this.

Passive stiffness of muscle–tendon units and their fascia comes back when you stop giving them regular, long passive loads. Meta-analyses show that static passive stretching done for 3 to 12 weeks reduces passive muscle stiffness. These reductions are measured by both shear modulus and passive torque, and the effect increases with greater total volume. ^[12] Without that loading, the tissues lose the input needed for viscoelastic stress relaxation and for collagen fibres to reorganise. As a result, the passive torque–angle curve shifts to the left over time. This means you feel stronger tension at the same joint angle than before. Active drills at long muscle lengths can increase fascicle length and improve active control close to end range. But these drills do not give the same slow, steady tensile strain to passive tissues that static stretching alone provides. ^[22]

Second, tolerance to stretching changes with repeated passive exposure. Both early and recent studies show that the increases in range after stretching come mainly from altered perception, not from lasting structural changes. ^[8] In practice this means that trainees allow more discomfort at the end range before they stop the stretch. Experiments on pain show that people with stronger natural pain inhibition gain greater improvements in tolerance. ^[11] If these passive exposures are stopped, the nervous system resets itself toward protective limits. As a result, the tolerated passive angle decreases, even though strength at long muscle lengths stays the same, or can even increase.

Third, there is considerable difference in sensorimotor prediction when you move into extreme positions by your own effort versus when force is imposed from outside. In active flexibility, your brain sends the motor commands. It increases γ drive and predicts the sensations that will follow. Because the brain expects what happens, there is less surprise, and reflex responses are weaker. In contrast, when you are pushed into a position by an outside force or by gravity, your brain does not send such commands. There is no efference copy, γ drive stays lower and muscle spindles report a stretch that was not predicted. In this case, Ia afferents drive stretch reflex pathways with greater force unless presynaptic inhibition is activated, while Ib feedback signals tendon tension without the matching sense of effort. The nervous system reads this mismatch as threat. Over time, if you do not desensitise through repeated practice, this bias makes passive end ranges less and less tolerable, while your active control remains. ^[23-25]

Fourth, history-dependent muscle mechanics have direct, practical importance. Thixotropic behaviour means that what you did before (your movement and loading history) determines the stiffness you feel right now. ^[26] If you repeat static passive holds at the end of your range, you affect the organisation of intramuscular cross-bridges and connective tissue. As a result, later sessions start with less passive resistance. But when you stop such exposures, the stiffness you feel at the start of training goes up. This limits the passive angle you can reach on that day and, by making you avoid it, increases your sensitivity.

These neural and mechanical explanations fit the results of flexibility training. Meta-analyses of stretching show that lasting gains in range appear only after at least two weeks of practice, regardless of method, and that static passive stretches of sufficient intensity can also produce small increases in fascicle length. ^[13] Strength training done in extended ranges, especially with an eccentric focus, develops range and adds fascicle length, but recent studies show these gains can come even without lowering passive mechanical stiffness. ^[6,27] This means that active training at long muscle lengths helps maintain or expand active range and muscle architecture, while skipping passive holds allows passive stiffness and the body’s sense of threat to grow. In turn, this shrinks the passive end range in positions such as the splits.

Clinically, we see that the division between active and passive mechanisms of tension detection has direct consequences for training. If the goal is to preserve extreme passive ranges, we must use repeated exposure to slow, static passive postures. This keeps adaptations in passive stiffness and maintains tolerance to stretch. To engage the γ drive and reduce reflexive reactivity near the end range, apply dynamic loaded stretching or isometric contractions. After that, add relaxed holds to guide the recalibration of sensory perception. Watch closely both the changes in passive torque and the subjective reports of discomfort. Together, these show the mechanical and central influences that set the limit at end range. Research shows the same: static passive stretching acutely modulates spinal excitability, chronically reduces passive stiffness and engages pain-modulatory systems that determine the limit of a tolerable range.

6. Counterarguments and limitations

In this article I have laid out a framework that classifies flexibility into four subtypes, a classification that gives a clearer and more detailed understanding than the usual division into passive and active flexibility. This multidimensional approach agrees with both biomechanical research and clinical observations. Still, I must point out several limitations regarding the definitions used, the physiological mechanisms involved and the strength of the evidence available.

First, the four categories are not applied consistently in research, which makes comparison and synthesis across studies difficult. For example, dynamic passive flexibility is almost never examined outside of rehabilitation. Researchers need to adopt a common vocabulary so that the same terms mean the same things to everyone. Without this, progress will remain slow.

Second, while I have described dynamic active flexibility as “flexibility expressed at speed,” a definition that is often used in athletic training, the actual mechanism is probably not flexibility itself. Rather, it is task-specific potentiation (the effects of increased temperature, greater motor unit recruitment and higher muscle compliance). This means that while it is helpful to explain dynamic active flexibility as related to static passive flexibility, we must not confuse short-term potentiation effects of dynamic stretching with long-term adaptations in range of motion. Treating the two as the same leads to false conclusions.

Current meta-analyses confirm the broad principles of this framework: short-term neural effects from dynamic methods, long-term increases in range of motion from stretching, resistance training as an alternative means to improve flexibility and context-dependent effects on performance. What is less clear is how well these findings transfer to functional performance and to clinical outcomes.

The correct way to interpret this framework is to recognise both sides: on the one hand, its value as a more precise way of understanding flexibility, and on the other hand, the significant gaps in research that remain to be filled.

7. Conclusion

Flexibility means the ability of a specific joint to move through its full range without excessive discomfort. It depends on two principal factors: the mechanical properties of tissues and the nervous system’s control of stretch tolerance. Flexibility is not a general quality for the whole body but a joint-specific ability. To measure it accurately, you must use tools that are specific to the joint and the task. Each tool has limits, and you must understand them. The usual division of flexibility into passive and active range of motion is too narrow. It does not account for whether the joint is static or dynamic. Muscle activity (passive or active) and joint action (static or dynamic) always happen together. At any time, a muscle is either passive or active, and the joint it moves is either held still or moving. For this reason, you must distinguish four kinds of flexibility: static passive, static active, dynamic passive and dynamic active.

Read the next article: Flexibility vs Range of Motion

Medical disclaimer

This content is for education purposes only. It does not replace medical advice. Do not start or change exercise or rehabilitation programmes without consulting a qualified healthcare provider.

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