Few beliefs in fitness are held as firmly as the idea that a tight muscle is a short muscle. It follows so neatly. The muscle is short, so you pull on it, and over time it gets longer. Every hamstring stretch anybody has ever been given rests on that one belief.
The problem is that it has been tested properly, and muscles do not get longer.
Across sixty-five studies and 1,542 participants, static stretching produced no change in fascicle length. Not after a single session, where the effect was 0.11 and non-significant. Not after weeks of training, where it was 0.07 with a p-value of 0.95 (Ingram et al., 2025). That is about as clear as sports science gets.
Range still improves. It improves a lot. So if the muscle has not changed length, something else has, and that something is largely sitting between the ears rather than in the tissue.
This is the idea the whole article is based on, and it explains something you may already have seen in your own clients. They are the ones who have stretched diligently for six months and who now sit far more comfortably in the stretch than they used to, yet whose movement under load looks no different to when they started.
A Muscle That Never Got Longer
Start with what does change, because the answer is more interesting than a flat “it is all in your head”.
The same analysis that found no length change measured two other things. The first was passive stiffness, meaning how much resistance the tissue offers as you take it through range. The second was maximum tolerable passive resistive torque, which is a technical way of describing the point at which the person says stop.
Acutely, within one session, the range gain is driven mostly by a real reduction in passive stiffness, at 0.42. The change in tolerance does not reach significance. Chronically, over weeks, the picture inverts. Tolerance becomes the dominant driver at 0.74, while the stiffness reduction shrinks to a junior partner at 0.37 (Ingram et al., 2025).
Let’s read that inversion again, because it is the useful bit. The single stretch you do in a warm-up is mostly mechanical. After six months of stretching is mostly neural. Most coaches assume the opposite.
So stiffness is largely regulated rather than fixed, and increasingly so the longer the timescale. Not purely regulated. The tissue effect is real and it should not be dismissed.
Range of Motion, Flexibility and the Words We Use Loosely
Before going further into mechanism, the vocabulary needs sorting out, because our industry has been using four words interchangeably for about thirty years.
A 2026 position piece in Sports Medicine put a stake in the ground. Range of motion is the umbrella term, and it has modifiable determinants such as soft tissue extensibility and neural control, plus non-modifiable ones such as bone shape and joint architecture. Flexibility is narrower than that. It is the extensibility of the soft tissues around a joint, and it is a trainable subset of range of motion. Flexibility training is also not the same thing as stretching, because stretching is one method among several that increases flexibility (Afonso et al., 2026).
The authors go further and name ACSM, NASM and NSCA as defining these terms inconsistently, with two of them treating flexibility training as though it were simply stretching.
And then there are the two words we use most. “Mobility” has no agreed operational definition in the research literature. “Movement capacity” has no definitional literature at all. Both are coaching language.
Neither word is being criticised here. Both are useful, and the new Movement & Mobility course uses them deliberately. But a coach who knows which of their terms are defined and which are shorthand is a coach who can explain themselves properly to a physiotherapist, and that is a professional advantage worth having.
What the Nervous System Is Doing
Here is the physiology in plain terms, and with the common errors stripped out.
Muscle spindles sit in parallel with the working fibres, wrapped around specialised fibres inside their own capsule. Group Ia afferents from the spindle signal the rate of length change, meaning how fast the muscle is being lengthened. Group II afferents signal the magnitude, meaning how long it currently is. Spindles are primarily velocity detectors with a length component, rather than simple rulers.
Golgi tendon organs sit in series at the junction between muscle and tendon. They sense force, not length. They are most active during contraction, and their fibres fire steadily through an isometric hold.
Alpha motor neurons drive the fibres that produce force. Gamma motor neurons do something different and more interesting. They innervate the small fibres inside the spindle and adjust how sensitive it is. They do not generate any useful joint torque. What they do is set the gain.
That last point IS the whole argument. Because gamma drive can be set independently, the same physical stretch can produce a different afferent signal depending on how sensitive the nervous system has set things at that moment. And because the strongest correlate of long-term range gain is the point at which somebody chooses to stop, the terminal limit in a healthy client is a perceptual and decisional one, arbitrated centrally.
Add in protective guarding, where the system raises muscle activity in response to actual or anticipated harm, and you have a plausible account of the client who is stiff in one context and moves freely in another. Nothing about their tissue changed between the two.
The Myths to Bust
Several things taught confidently on this subject do not standup against the evidence.
“The stretch reflex is what stops you going further” is wrong for sustained static stretching in a healthy person, where muscle EMG is essentially silent. The terminal limiter is tolerance plus passive tissue resistance, not reflex activity.
“Golgi tendon organs detect dangerous stretch and switch the muscle off” is wrong twice over. They detect force rather than length, and they are most active while the muscle is contracting.
“PNF works through autogenic inhibition” does not hold up either. The timing does not work, because the inhibitory effect is short-lived and has decayed before the post-contraction stretch is performed. A 2025 elastography study ran six sets of contract-relax stretching on the rectus femoris and found no change in muscle stiffness at all, at any dose. Something is happening during PNF, and it appears to be a change in perception rather than in tissue.
“Reciprocal inhibition switches the antagonist off” overstates a real spinal circuit. It modulates excitability. It does not produce relaxation on demand, and in a healthy person the muscle being stretched is already electrically quiet.
None of this makes stretching pointless. It relocates where the effect is coming from.
Where Stretching Stops Paying Off
If tolerance is the main long-term mechanism, then more stretching should keep buying more tolerance. It does not, and the ceiling is lower than you would expect.
A meta-regression across 189 studies and 6,654 adults found that the benefit plateaus at roughly four minutes per session acutely, and roughly ten minutes per week per muscle group chronically. Intensity, age, sex and training status did not moderate the outcome. Poor baseline flexibility predicted larger gains, which is intuitive (Ingram et al., 2025b).
Ten minutes a week, per muscle group. The client doing forty-five minutes of daily stretching is spending an enormous amount of time to buy almost nothing beyond that ceiling, which is precisely why the diligent stretcher plateaus and then decides they are just not flexible.
The coaching move is not to prescribe more. It is to spend the time somewhere else.
The Overstated Performance Warning
While we are retiring things, the other claim to revisit is that static stretching before training ruins performance.
A multilevel meta-analysis of eighty-three studies, more than 400 effect sizes and 2,012 participants found the isolated maximal strength loss to be small overall at −0.21. Holds of sixty seconds or longer against a passive control produced a large deficit at −0.84. But compared against an active control, which is the fair comparison, the effect shrank to somewhere between −0.17 and −0.28, meaning a good deal of what looked like a stretching cost was simply fatigue. Athletic performance was not impaired, and subsequent jump performance actually improved slightly at 0.15 (Warneke and Lohmann, 2024).
The authors state directly that the results do not support previous recommendations to exclude static stretching from warm-ups before jumping or sprinting.
Below sixty seconds per hold, the strength effect is around −0.07, which is nil. Nobody in a normal warm-up is holding one stretch for over a minute or accumulating eight minutes of static work. The warning was extrapolated from laboratory protocols that almost nobody uses.
The Passive to Active Gap
Now to the thing that actually limits clients.
Most people have more passive range than they can use. Lie them down and lift the leg and it goes a long way. Ask them to lift it themselves and it stops much earlier. The territory between those two points is range they own on paper and cannot access under their own control.
Passive stretching raises the ceiling. It does very little about the gap, because tolerance to being taken somewhere is not the same as the ability to get there and produce force once you get there.
What closes the gap is load at length. Full-range resistance training produces a flexibility effect in the same ballpark as stretching, with the useful difference that it builds strength through the range at the same time. And the 2026 terminology paper states clearly that strength training at long muscle lengths produces chronic range gains comparable to stretching (Afonso et al., 2026).
One caveat. The passive-to-active gap is a sound programming principle, not a validated measurement. There is no agreed protocol for quantifying it and no normative data, and the popular claim that a large gap predicts injury has no prospective evidence behind it that I can find. Teach it as a way of organising your thinking, not as a diagnostic number.
What Actually Increases Flexibility
There is one method that produces structural change, and it is not stretching.
A single-blind randomised trial put twenty-seven physically active adults through eight weeks and twenty-four sessions of eccentric plantar flexor training. One group trained at long muscle length, taking the ankle from neutral into around thirty degrees of dorsiflexion. The other trained the same movement at short muscle length. Fascicle length increased by 8.5 per cent, around 3.4 millimetres, in the long-length group only. The short-length group showed no change. Both groups gained about 9.5 per cent in maximal isometric strength (Bizet et al., 2025).
Passive stiffness and slack angle did not change in either group. The muscle rebuilt itself without the tissue becoming more compliant.
Put the two findings side by side. Eight weeks of stretching changes fascicle length by nothing. Eight weeks of heavy eccentrics at long muscle length changes it by 8.5 per cent. Load at length is the stimulus. Duration of hold is not.
What This Means for a Programme
| Method | What Actually Changes | Best Use and Dose |
|---|---|---|
| Static stretching, in-session | A transient reduction in passive stiffness. Tolerance change is not significant this quickly. No change in fascicle length | Opening range for the session ahead. Under 60 seconds per hold and under 8 minutes in total, below which there is no strength cost at all |
| Static stretching, across weeks | Mostly increased stretch tolerance, with a smaller but real stiffness reduction alongside it. Still no change in fascicle length | Raising the passive ceiling in a client who is properly range-limited. Around 10 minutes per week per muscle group. Gains plateau beyond that |
| Dynamic stretching | Raises available range with no strength cost, and prepares the system rather than changing the tissue | The default choice before training, and the obvious pick when the next thing is a maximal effort |
| PNF and contract-relax | Not autogenic inhibition. Six sets produced no measurable change in muscle stiffness. The effect runs through perception and tolerance | When a partner is available and the client responds well to it. Choose it on preference rather than on a claimed mechanism |
| Full-range resistance training | Real flexibility gain in a similar ballpark to stretching, plus force capacity through the range | The efficient default for general clients. One intervention, two outcomes. Load it properly |
| Eccentrics at long muscle length | The only method here with demonstrated architectural change, at 8.5 per cent fascicle length over 8 weeks, plus a 9.5 per cent strength gain | Closing the passive to active gap in a client with range they cannot control. Around 24 sessions across 8 weeks |
| Strength and control for the hypermobile client | Proprioception and control, rather than more range. These clients already have surplus passive range | Any client with generalised joint hypermobility. Coach it as a control problem |
Pain, Threat and Where Our Job Ends
Some stiffness is protective. The system has decided that a direction of movement is risky and has raised the background tone accordingly. Pulling harder on a muscle that is being guarded is a poor strategy, because the guarding is a response to threat rather than a property of the tissue.
Two things follow for a coach.
The first is that reducing the perceived threat is part of the work. Predictable sessions, graded exposure, ranges the client can succeed in, and explaining what is happening rather than leaving them to assume the worst. The evidence on pain education is that it works best alongside movement rather than instead of it, and that its most consistent effect is on fear of moving rather than on pain itself. For a trainer that is a good division of labour, because the movement is our part.
The second is scope. CIMSPA’s professional standard for personal trainers requires you to use recognised screening, to understand the limits of your role and to know when to signpost or refer. Explaining that stiffness is regulated rather than fixed sits inside that. Diagnosing why somebody hurts sits outside it. Coaching graded exposure to range is ours. Managing acute, severe, worsening or unexplained pain is not, and neither is anything accompanied by night pain, unexplained weight loss, neurological signs or a recent injury.
The Hypermobile Client
Roughly the opposite problem, and one that can easily be missed.
The Beighton score is the screen most people use, and its limitations are considerable. A UK review from Warwick Medical School and the Royal College of General Practitioners found no evidence-based justification for the familiar four-out-of-nine cut-off. The score is all-or-nothing, giving no indication of severity, and it does not specify whether active or passive range is being assessed. Around two-thirds of the scored items are upper limb, and hips, shoulders, ankles and most of the axial skeleton are not assessed at all. Shoulder hypermobility, often the most symptomatic site, shows no correlation with the score (Malek et al., 2021).
The authors’ recommendation is that a negative Beighton score should not be used to rule hypermobility out.
A screening tool is all it is, and screening is not diagnosis. Hypermobility spectrum disorder and hypermobile Ehlers-Danlos syndrome are diagnosed clinically, the 2017 international criteria still stand, and a revision from the international consortium is expected toward the end of 2026.
What the coaching should look like is easier. A hypermobile client already has more passive range than they can control. Adding more of it adds nothing they can use. What they lack is the ability to generate and regulate force across the range they already have, so the programme is loaded work at end range, position awareness and control, not another twenty minutes of stretching.
Where Joint by Joint Came From
The joint-by-joint model, in which joints are said to alternate between needing mobility and needing stability, is everywhere in coaching education.
It came from Michael Boyle and Gray Cook. It is a coaching heuristic, and there is no primary research testing the alternating pattern, no validation study and no outcome trial. The nearest peer-reviewed relative is regional interdependence, the observation that impairment in one region relates to symptoms elsewhere, and that literature is largely clinical and observational.
The newest direct look at this is not especially kind to the strong version of the claim. A 2026 study measured active range across four myofascial chains in sixty-one healthy adults and found correlations between adjacent regions running from about 0.29 to 0.60, concluding that the interdependence is region-specific and inconsistent rather than uniform.
Use it as a map. It is a handy way to organise an assessment and to stop you looking only at the joint that hurts. Just do not present it to a client, or to a physiotherapist, as how the body is built.
Bringing It Into Your Coaching
Many of us find our clients arrive with a stretching routine and a theory about why it is not working. The theory is almost always that they need to do more of it.
As we have seen, the evidence points somewhere else. Range improves mostly because the nervous system permits more of it, tissue length is largely unmoved by stretching, the ceiling on stretching volume is around ten minutes a week per muscle group, and the only method with demonstrated architectural change is loading at long muscle lengths.
Practically that reshapes a session. Keep stretching in. It works over time, but the gains are not about how much of it you do. Spend the time you free up on strength through full range, on eccentric work at long lengths and on control at the end of range the client already owns. For the hypermobile client, invert the whole thing.
And explain things. A client who understands that their stiffness is a setting rather than a fixed length will train differently in a session, and get a better result than any stretching protocol claims to.
References
Afonso, J., Blazevich, A.J., Behm, D.G., Tilp, M. and Warneke, K. (2026). One of These Things Is Not Like the Others: Disentangling the Concepts of Range of Motion Versus Flexibility, and Flexibility Training Versus Stretching. Sports Medicine. Click here to review the full research article.
Bizet, B., Nordez, A., Tallio, T., Lacourpaille, L., Cattagni, T., Colard, J., Betus, Y., Dorel, S., Sarcher, A., Seynnes, O. and Andrade, R.J. (2025). Eight Weeks of Eccentric Training at Long-Muscle Length Increases Fascicle Length Independently of Adaptations in Passive Mechanical Properties. Journal of Applied Physiology. Click here to review the full research article.
Ingram, L.A., Tomkinson, G.R., d’Unienville, N.M.A., Gower, B., Gleadhill, S., Boyle, T. and Bennett, H. (2025a). Mechanisms Underlying Range of Motion Improvements Following Acute and Chronic Static Stretching: A Systematic Review, Meta-Analysis and Multivariate Meta-Regression. Sports Medicine, 55(6), pp.1449-1466. Click here to review the full research article.
Ingram, L.A., Tomkinson, G.R., d’Unienville, N.M.A., Gower, B., Gleadhill, S., Boyle, T. and Bennett, H. (2025b). Optimising the Dose of Static Stretching to Improve Flexibility: A Systematic Review, Meta-Analysis and Multivariate Meta-Regression. Sports Medicine, 55(3), pp.597-617. Click here to review the full research article.
Malek, S., Reinhold, E.J. and Pearce, G.S. (2021). The Beighton Score as a Measure of Generalised Joint Hypermobility. Rheumatology International, 41, pp.1707-1716. Click here to review the full research article.
Warneke, K. and Lohmann, L.H. (2024). Revisiting the Stretch-Induced Force Deficit: A Systematic Review with Multilevel Meta-Analysis of Acute Effects. Journal of Sport and Health Science, 13(6), pp.805-819. Click here to review the full research article.
Train the Way Movement Actually Works
Everything above is the reason we built a new course, and it launches this week.
The Movement & Mobility Course follows exactly the ground this article covers. The difference between mobility, flexibility and movement capacity. How the nervous system governs stiffness. Pain science, hypermobility and instability. The types of stretching and what each one is really doing. Programming to close the passive to active range gap, and a joint-by-joint approach to assessing range. It is CIMSPA recognised and studied by distance, with a video practical assessment, so it fits around a full client diary.
Introductory Offer: Get started on the UK's first Movement & Mobility course with 10% off. Normally from £449. Today from just £399 or from £75/month. Offer Ends 21st August!
Movement & Mobility Course – Distance Study
