Stretching has a place of honour in athletics, sports medicine, and fitness — it is an activity everyone loves to love — and yet it is arguably the most over-rated thing you can do with a body. Most of the reasons athletes stretch — warming up and injury prevention, recovering from soreness, enhancing performance — do not stand up to scientific scrutiny.
There is really only one effect of stretching that seems to be clear and (almost) uncontroversial: it does actually increase flexibility. Even just plain old “static” stretching, and not even that much of it. Just putting tension on a muscle will do the job.
For whatever it’s worth, people do seem to be more flexible when they stretch regularly for a while. Meaningful improvement is elusive, but it can be done. The phenomenon is widely observed, and seems to have been confirmed by experiments, several of which will be cited below. But less than you’d think at this point in history! There’s hundreds of studies of anything these days, but not this.
So, you almost certainly can get more flexible “for whatever it’s worth,” but … what is it worth? Exactly? When you try to spell out it out, it’s surprising how quickly things get a bit “um, er, well.”
The value of flexibility is the main focus of this article. For a bird’s eye view of the whole topic stretching, see Quite a Stretch. For information about stretching as a form of self-treatment for pain, see Stretching for Pain (and Pleasure). For more about stretching for sports and fitness — especially injury prevention and treatment — see 5 Main Reasons Athletes Stretch… All Flawed.
But for the science of flexibility … just keep scrolling.
The American College of Sports Medicine believes that flexibility is “important in athletic performance (e.g., ballet, gymnastics) and in the ability to carry out activities of daily living.” It’s a “major component” of fitness, along with body composition, cardiovascular endurance, muscle endurance, and muscle strength. They recommend 2–3 days of stretching per week (2–4 repetitions of multiple stretches per day).1
Personal trainers certainly buy into it. About 80% of American trainers prescribe static stretching — for all the reasons, but mainly for the flexibility.2
I will start by arguing that is not worth much to most people. Even athletes. Even athletes who supposedly need extra flexibility.
To make that case more effectively, it’s also important to consider the nature of flexibility. When someone increases their flexibility, what changes, exactly? How does it work? Contrary to what nearly everyone believes, it’s probably not by actually making tissue “longer.” And so I’ll close the discussion by looking at the fascinating topic of plasticity versus neurological tolerance.
Flexibility has been researched for over 100 years. Its track record is unimpressive, particularly when viewed in light of other components of physical fitness. Flexibility lacks predictive and concurrent validity value with meaningful health and performance outcomes. Consequently, it should be retired as a major component of fitness.
~ Nuzzo, 2020, Sports Medicine
“I want to be more flexible,” people say. Even when they have normal range of motion in every joint. What’s this about? Why are people so determined to be more bendy? What is it you want to do with that super power?
The fact that there are actually several elastic superheroes speaks to our genuine craving for greater range of motion. We love the idea. But the reality is that hardly anyone actually needs to be more flexible. Most people have a normal range of motion — that’s why it’s normal! Unless you are specifically frustrated because you lack sufficient range of motion in a joint to perform a specific task, you probably don’t need to be more flexible.
What exactly are you planning to do with your flexibility?
Even abnormally poor flexibility is usually no big deal, as long as you are otherwise healthy. There just aren’t many functional limitations caused by being super stiff that actually matter much. A couple examples:
- Toe touching is the ultimate in trivial flexibility goals. People can live their whole lives without being able to touch their toes without any practical consequences, for instance. This is one of the key points of the article: that flexibility should only matter if it can help us do something that actually matters to us. For the martial artist, flexibility enables performance goals that martial artists care about! Great! But if a dude doesn’t care if he can touch his toes ... so what?
- Reaching bra buckles actually matters to women, of course, but — unless something is diagnosably wrong with that shoulder — few women are so inflexible that it’s an issue. If it was, they would certainly consider it well worth fixing -- et voila, a job for stretching! But it's both rare and easy to fix: it wouldn’t take much improvement in shoulder ROM.
For stretching with a flexibility goal to matter, we have to identify a common and meaningful functional limitation that can be overcome by increasing range of motion. There are lots of examples that are rare and trivial ... or common and trivial ... or rare and meaningful ... but not both common and meaningful. And so the seemingly worthy goal of better ROM is actually quite marginalized — it doesn't and shouldn't matter to most people, most of the time.
If inflexibility is good enough for Eliud Kipchoge, it’s good enough for you. As of 2018, Mr. Kipchoge is the world’s fastest marathoner.3 But the man cannot touch his toes! This is an anecdote from someone who trained with him leading up to when he broke that record:
He was extremely inflexible. After an easy morning run (16km in 1hr10mins), I stretched with the group. Most of them were fairly flexible in the hamstrings (ie. standing straight legged and bending over to touch your toes with your knees straight and being able to touch your toes. Eliud was miles off). He was nowhere near touching his toes! They all found it hilarious that he couldn’t come close to touching his toes.
Matt Fox for SweatElite.co, “Training with Eliud Kipchoge – 5 Things That Surprised Me”
When people have good aerobic capacity — when they are fit enough to efficiently distribute oxygen to all their cells — they are measurably more functional and healthier in all kinds of other ways too. If stretching is also an important part of fitness, then it too should be linked to all kinds of other good stuff.
It’s not. Stretching barely registers on any measure of health and function. More flexible people do not die less or fall down less as they age. They don’t have clearly higher quality of life (a tough one to measure, but it’s been done). They don’t have less back pain or injuries, and in fact they may have more. They don’t have lower blood pressure, resting heart rate, cholesterol — those markers of fitness are much more strongly predicted by weight and endurance.4
Flexibility also isn’t linked to the other components of fitness: flexible people cannot lift more, run farther, or slip through narrower cracks. James Nuzzo: “Absence of correlations between flexibility and other fitness components indicates flexibility is a distinct trait, but not one particularly important for health and function.”
What if flexibility really did matter to you, for some reason, doesn’t matter what? Could you do it?
It’s not necessarily easy, or good bang for buck, and it may depend on your genes.
I really can’t seem to do it, which established long ago with a really thorough personal experiment. Many other people feel defeated by this challenge as well. Here’s a story sent in by a reader who has been working on it for decades:
I have done yoga every day more than 20 years. I'm really devoted to it. I do different set of positions every 2 weeks, but I start my sessions every day with 5-10 minutes of breathing in simplified/easier lotus position (padmasana, where one foot is on the floor, and another is on top of the opposite thigh). It is still just as uncomfortable now as it was when I started. I can handle it for those 5-10 minutes, but more is just too hard. I can still do the full version, but only for a minute or so. One would hope after so many years, doing it daily, I would become more flexible! But not even a little bit — I do the yoga for other reasons.
Franjo M, California
However, for many people, a diligent effort over a period of weeks might well increase your range of motion. How much of an effort? That’s the next topic.
Hold it right there! For how long exactly, though? And how often? How hard? What works best? Is 5 seconds of light stretch every other day for a couple weeks enough? Probably not. Or should it be 5 minutes of brutally intense yanking on your muscles three times a day for three months? That’s probably too much — even if it was definitely more effective, ain’t nobody got time for that. The bottom line:
- no one really knows
- a reasonable amount is probably good enough
- the returns probably diminish steadily as the dosage increases.
In practice, most people stretch only a small selection of tissues quite briefly, seconds only, and erratically. Only the most diligent of us can actually sustain a daily habit for long. We don’t know if that’s enough to achieve the goal. The evidence suggests that holding a stretch for 15 seconds is better than 5… but only a little bit.10
In 2011, a nicely done experiment by Marshall et al showed that regular hamstring stretching substantially increased range of motion in normal university kids.11 Specifically, after “a 4-week stretching program consisting of 4 hamstring and hip stretches performed 5 times per week,” their range increased about 16˚ or 20%. That’s a real result from a sane stretching dosage. For whatever it’s worth.
Beliefs about flexibility and the optimal means of stretching have often proceeded from assumptions that have never been tested and from an almost religious zeal regarding the perceived benefits of stretching by a few.
~ Flexibility, by William Sands, p. 389
Here is a vivid example of how poorly understood this all is. This is an excerpt from one of my school text books, a weighty and authoritative tome, a bible of therapeutic exercise (granted, out of date now, but this is the text I learned from):
Several authors have suggested that a period of 20 minutes or longer is necessary for a stretch to be effective and increase range of motion when a low-intensity prolonged mechanical stretch is used.
three citations listed, Therapeutic Exercise, 3rd Ed., Kisner/Colby, p157
Twenty minutes?! Almost no one is stretching for that long. No one’s recommending that. And yet “several authors” have found that it is “necessary.” So it would seem to be a “correct” method of stretching, yet it is absent from professional wisdom on the subject … because, of course, it is contradicted in other text books, by other experts, not to mention the fact that it’s extremely impractical. Imagine trying to stretch for injury prevention: 20 minutes for each of even just 10 important muscles would be more than three hours. Again, ain’t nobody got time for that.
The idea of a “correct” dosage (or any aspect of technique) is mostly a fantasy. And yet, somewhere between “not nearly enough” and “way too much” there is probably a sweet spot.
Acrobats, gymnasts, yogis, contortionists, and martial artists have clearly been pushing the limits for centuries, sometimes achieving uncanny mobility. But these are highly motivated athletes with specific and exotic performance goals and stretching regimens that would definitely intimidate the rest of us, and with good reason: they often injure themselves along the way. Indeed, it may even be necessary to injure joints — to traumatize their capsules and ligaments — in order to get them to move that far.
Even these unusual athletes are not chasing flexibility alone, and most athletes have much higher priority training goals. “Athletes usually don’t require extreme ranges of motion,” writes Todd Hargrove,12 “but rather extreme control at the end of relatively normal ranges of motion.”
Fitness and health are not equivalent. You can be fit for a particular athletic pursuit, but that doesn’t mean you are a healthier person: high performance in a narrow category often comes at great costs (such as joint stability). Flexibility is good for a few specialized tasks … and really not much else. It’s useful for gymnasts, for instance.
Is this a good idea? This is an “oversplit.” Regularly doing this kind of thing will certainly make a person flexible … but not “healthy.”
Is this child gymnast going to be more flexible? Oh, yes, I think she will be! Will she perform better? Not necessarily: she might be better off doing strength training at the limits of her range — which will not only increase her range, but improve her control at the limits of her range. Is she going to be “healthier” for it? Blatantly not. And she has a middle-aged guy sitting on her back … and that’s just gotta suck.
Several explanations for increased flexibility from stretching have been proposed, and none have panned out. A 2010 paper in Physical Therapy reviews them all in great detail, and the full text is free.13 It’s not light reading, but there are some fascinating highlights. For instance, the authors torpedo the popular theory that muscles actually change length (“plastic deformation”):
In 10 studies that suggested plastic, permanent, or lasting deformation of connective tissue as a factor for increased muscle extensibility, none of the cited evidence was found to support this classic model of plastic deformation.
After reviewing several more disproven popular theories, they get to the good part: the last theory standing.
Increases in muscle extensibility observed immediately after stretching and after short-term (3 to 8-week) stretching programs are due to an alteration of sensation only and not to an increase in muscle length. This theory is referred to as the sensory theory throughout this article because the change in subjects’ perception of sensation is the only current explanation for these results.
Note the very interesting phrasing, “the only current explanation.” That’s a Sherlock Holmesian way of putting it: “Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.” It’s a strange, cool, and unexpected conclusion … but it’s also all we’ve got left, so we should probably take it seriously.
Of course, no debate is ever really over, and scientific evidence continues to accumulate. But stretch tolerance probably has a lot to do with flexibility gains.
Increased flexibility may simply be an increased tolerance for the discomfort of excessive muscle elongation.
If you do some upper body stretching, your lower body will get a little more flexible. Not much, but enough to measure in a controlled test.14 But that’s the only effect — which suggests that it’s due to an increase in tolerance of stretch, and that’s all. It’s one of many clues suggesting that stretch tolerance is the secret sauce in flexibility.
In other words, muscle (probably) doesn’t change, especially in response to an average stretching regimen … but our willingness to elongate it probably does. This is clearly supported by some research.15
Stretching is a way of teaching the nervous system that it’s okay to stretch a little further.
If so, then elongation must normally be limited by a strict neurological edict. The brain and spinal cord decree: you’re only going to lengthen your muscles so far, period, end of discussion. It’s not a negotiation … at least not in the short term. Don’t make the mistake of thinking you could just blast through that barrier with will power.
There is a strong analogy here to strength: we always have much greater muscle power available than we can safely use. We have deep reserves that are literally impossible to tap into on short notice, without large squirts of adrenalin. Contractions are normally reined in by the brain. Even with a powerful grunt of effort, only a small fraction of your muscle fibres get a signal to contract at any one time. If you recruited all of them, you might rip the muscle off your bones, or at least completely exhaust yourself in seconds. Your central nervous system has excellent reasons for imposing a power limit. Full contraction is for dramatic, obvious, life and death situations only.
However, with training, we can learn to recruit more fibres. In fact, when people train their muscles, early strength gains may be mainly a matter of learning to “recruit” more muscle fibres at once.
Here’s an interesting example from science of how increasing flexibility may be more of a nervous system “hack” than a matter of changing tissue. It appears that if you just add some vibration, even already flexible gymnasts can get a surprising boost in flexibility.161718 Clearly that is a neurological effect on flexibility … and a very cool one. (Vibration is an interesting topic: see Vibration Therapy.)
Despite all of the above, Team Plasticity remains large and devout: many professionals still believe that tissues adapt their structure to stretching. They can point to some research to support that position. The 2011 study of flexibility I cited above is great example (Marshall et al). It clearly showed that stretching increases flexibility: when subjects were stretched with the same force (torque) applied, pushed to the same level of discomfort, they could go 20% farther. So we know something changed! But … what? Was is a change in tissue? Or tolerance?
This data doesn’t say. But the authors seemed to think it did. They followed their data into an overinterpretation, presumably trying to score points for Team Plasticity. Because range increased, but pain at the end of the range did not, they unwisely concluded that a change in tolerance was not a factor.19 But stretching farther without hurting more could mean an increase in (neurological) tolerance! Although it wasn’t measured, it’s safe to assume the subjects’ pain would have been less if stretched only to the end of their original range.
This experiment was agnostic about mechanism. It cannot actually settle the bet — it demonstrated only a reduction in stiffness, but not whether it was due to neural or structural adaptations. And that’s the problem with the research as a whole: plasticity has almost been ruled out by a bunch of studies — here’s another fresh one20 — but not completely, and meanwhile neurological adaptation still hasn’t been confirmed.
Body is not stiff, mind is stiff.
~ K. Pattabhi Jois
As with contraction, your body probably has excellent reasons for strictly limiting elongation. When a stretch becomes uncomfortable, that’s your nervous system saying, “No way, sister, we don’t go there — we’ve got some sensible rules about this.”
And you really just can’t overrule your spinal cord on this. Talk about wisdom of the body!
But apparently we can get used to stretching — we can learn to tolerate greater elongation to some extent. Fascinating! This goes a long way to explaining the flexibility feats of yogis and martial artists, whose hypermobility might well be dangerously dysfunctional if it were attributable to plastic deformation. Plastic deformation simply does not occur in the most athletes, and maybe none. It might occur at the extremes of flexibility performance, but only so much — if you actually deformed your muscles and tendons enough to really preztel yourself, they would probably also be too loose to be useful the rest of the time.
It’s a tidy, attractive theory that plastic deformation is minimal, and contortionism largely powered by extremes of stretch tolerance — they have trained themselves to allow their latent capacity for full muscular elongation, but their muscles retain the ability to return to a normal length.
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About Paul Ingraham
I am a science writer in Vancouver, Canada. I was a Registered Massage Therapist for a decade and the assistant editor of ScienceBasedMedicine.org for several years. I’ve had many injuries as a runner and ultimate player, and I’ve been a chronic pain patient myself since 2015. Full bio. See you on Facebook or Twitter.
Specifically about stretching:
- Stretching Injury — How I almost ripped my own head off! A cautionary tale about the risks of injury while stretching
- A Stretching Experiment — What happens when you stretch your hamstrings intensely for several minutes a day in a steam room?
- IT Band Stretching Does Not Work — Stretching the iliotibial band is a popular idea, but it’s very hard to do it right, and it’s probably not worth it
- Stretching for Pain (and Pleasure) — Is stretching a treatment for chronic pain?
- 5 Main Reasons Athletes Stretch… All Flawed — Stretching science shows that it doesn't do most of what we hope it does
- Mobilize! — Dynamic joint mobility drills are an alternative to stretching that “massage with movement”
- Quite a Stretch — Stretching science has shown that this extremely popular form of exercise has almost no measurable benefits
- The Unstretchables — Eleven muscles you can’t actually stretch hard (but wish you could)
This article was originally part of a much larger article dating back to the first version in August 2000. Twenty years later, I split that big ol’ article up in August 2020, giving this sub-topic new life on its own page.
Aug 7, 2020 — Editing: Content is like-new after being extracted from stretching mega-article and converted into a stand-alone article on the sub-topic of athletic goals for flexibility.
February — More information: Added detail about the practical importance of flexibility. Or lack thereof. Weirdly, this update was harvested from a decade-old Facebook comment. [Updated section: The value of flexibility.]
2019 — New section: Some new content, and some old content transplanted from other sections. There’s certainly more to do on dosage, but I think it’s a solid summary so far. [Updated section: Stretching dosage for flexibility: intensity, frequency, and duration.]
2019 — New section: Some new content, and some old content transplanted from other sections. There’s certainly more to do on dosage, but I think it’s a solid summary so far. [Updated section: Is flexibility actually even a benefit?]
2019 — Science update: Added five citations showing that stretching can increase flexibility. [Updated section: The value of flexibility.]
2018 — Minor addition: Added an anecdote from the world of elite distance running, about Eliud Kipchoge’s inability to touch his toes. [Updated section: The value of flexibility.]
- American College of Sports Medicine. ACSM’s guidelines for exercise testing and prescription. 10th ed. Philadelphia: Wolters Kluwer; 2018.
- Waryasz GR, Daniels AH, Gil JA, Suric V, Eberson CP. Personal Trainer Demographics, Current Practice Trends and Common Trainee Injuries. Orthop Rev (Pavia). 2016 Sep;8(3):6600. PubMed #27761219 ❐ PainSci #52598 ❐
- 2:01:39, which is 1:18 faster than the previous record, the greatest improvement since 1967. This dude is fast.
- That’s a lot of claims. Rather than citing to support all of them, I’ll just refer you to Nuzzo’s very detailed review, previously cited, which cites many examples and also acknowlgedes some exceptions. The exceptions, however, mostly just “prove the rule.”
- Freitas SR, Mendes B, Le Sant G, et al. Can chronic stretching change the muscle-tendon mechanical properties? A review. Scand J Med Sci Sports. 2018 Mar;28(3):794–806. PubMed #28801950 ❐ “Stretching interventions with 3- to 8-week duration do not seem to change either the muscle or the tendon properties, although it increases the extensibility and tolerance to a greater tensile force.”
- Behm DG, Blazevich AJ, Kay AD, McHugh M. Acute effects of muscle stretching on physical performance, range of motion, and injury incidence in healthy active individuals: a systematic review. Appl Physiol Nutr Metab. 2016 Jan;41(1):1–11. PubMed #26642915 ❐ PainSci #52916 ❐ “All forms of training induced ROM improvements, typically lasting <30 min.”
- Medeiros DM, Cini A, Sbruzzi G, Lima CS. Influence of static stretching on hamstring flexibility in healthy young adults: Systematic review and meta-analysis. Physiother Theory Pract. 2016 Aug;32(6):438–445. PubMed #27458757 ❐ “In all tests, the results favored static stretching compared to control group… In conclusion, static stretching was effective in increasing hamstring flexibility in healthy young adults.”
- Decoster LC, Cleland J, Altieri C, Russell P. The effects of hamstring stretching on range of motion: a systematic literature review. J Orthop Sports Phys Ther. 2005 Jun;35(6):377–87. PubMed #16001909 ❐
Results were inconclusive as to whether or not any particular type of stretching could improve knee ROM. Overall, however, “the evidence appears to indicate that hamstring stretching increase range of motion,” regardless of the technique used. “Appears to indicate” is pretty inconclusive.
- Harvey L, Herbert R, Crosbie J. Does stretching induce lasting increases in joint ROM? A systematic review. Physiother Res Int. 2002;7(1):1–13. PubMed #11992980 ❐ “The results of four 'moderate' quality studies show a convincing effect of stretching in people without functionally significant contracture.”
- Roberts JM, Wilson K. Effect of stretching duration on active and passive range of motion in the lower extremity. Br J Sports Med. 1999 Aug;33(4):259–63. PubMed #10450481 ❐
Another (small) study to determine if stretching for five or fifteen seconds can make a difference in range of motion of the joint. It concluded: “These findings suggest that holding stretches for 15 seconds, as opposed to five seconds, may result in greater improvements in active ROM. However, sustaining a stretch may not significantly affect the improvements gained in passive ROM.” In other words, the returns diminish quickly.
- Marshall PW, Cashman A, Cheema BS. A randomized controlled trial for the effect of passive stretching on measures of hamstring extensibility, passive stiffness, strength, and stretch tolerance. J Sci Med Sport. 2011 Nov;14(6):535–40. PubMed #21636321 ❐
- Hargrove T. Playing With Movement: How to explore the many dimensions of physical health and performance. 1st ed. self-published; 2019. p. 148
- Weppler CH, Magnusson SP. Increasing muscle extensibility: a matter of increasing length or modifying sensation? Phys Ther. 2010 Mar;90(3):438–49. PubMed #20075147 ❐ PainSci #55283 ❐
- Behm DG, Cavanaugh T, Quigley P, et al. Acute bouts of upper and lower body static and dynamic stretching increase non-local joint range of motion. Eur J Appl Physiol. 2016 Jan;116(1):241–9. PubMed #26410819 ❐
In this study, range of motion and strength were tested in the upper body after static stretching in the lower body, and vice versa. Passive range of motion was modestly improved, which is nifty, but not active ROM or strength, suggesting that “enhanced stretch tolerance was likely the significant factor,” as opposed to a mechanical or neural drive mechanism.
- Blazevich AJ, Cannavan D, Waugh CM, et al. Range of motion, neuromechanical, and architectural adaptations to plantar flexor stretch training in humans. J Appl Physiol (1985). 2014 Sep;117(5):452–62. PubMed #24947023 ❐
For this study, several people stretched their calf muscles twice per day for three weeks (each session consisted of 4 stretches of 30 seconds). They were compared to people who did not stretch. Measurements included changes in muscle and tendon mechanics, muscle activity, and spinal motoneuron excitability. The non-stretchers didn’t change; stretch training “elicited a 19.9% increase in dorsiflexion range of motion (ROM) and a 28% increase in passive joint moment at end ROM.” However, the ROM improvement was not explained by changes in the muscle structure: “Thus, increases in end ROM were underpinned by increases in maximum tolerable passive joint moment (stretch tolerance).
- Issurin VB, Liebermann DG, Tenenbaum G. Effect of vibratory stimulation training on maximal force and flexibility. J Sports Sci. 1994 Dec;12(6):561–6. PubMed #7853452 ❐
In this 1994 experiment, as described by Sands et al, gymnasts “used a vibrating ring suspended by a cable, in which the foot of the subject was placed while they stretched forward over the raised leg, targeting the hamstrings. The resulting increase in ROM was astonishing. These researchers demonstrated that vibration could enhance flexibility.” The results were replicated by Sands et al in 2006, and Kinser et al in 2008.
- Sands WA, McNeal JR, Stone MH, Russell EM, Jemni M. Flexibility enhancement with vibration: Acute and long-term. Med Sci Sports Exerc. 2006 Apr;38(4):720–5. PubMed #16679989 ❐
This experiment replicated the results of an intriguing 1994 experiment by Issurin et al. Ten highly trained gymnasts did forward splits with or without vibration. They stretched to the point of discomfort for 4 minutes, alternating between each leg, 10 seconds of stretching at a time. Flexibility immediately after stretching with vibration was dramatically greater; the long-term results were less striking.
- Kinser AM, Ramsey MW, O’Bryant HS, et al. Vibration and stretching effects on flexibility and explosive strength in young gymnasts. Med Sci Sports Exerc. 2008 Jan;40(1):133–40. PubMed #18091012 ❐
- Several things in the paper suggest their beliefs about the mechanism of increased flexibility, but it’s clearest here: although they concede that they “cannot completely rule out volitional stretch tolerance as a possible explanation for changes in extensibility,” they still conclude that “it does seem that hamstring pain elicited during a passive stretch has little involvement in explaining training related improvements.” Really? Sure seems to me like it could!
- Konrad A, Tilp M. Increased range of motion after static stretching is not due to changes in muscle and tendon structures. Clin Biomech (Bristol, Avon). 2014 May 9. PubMed #24856792 ❐
Another data point in the plasticity vs. tolerance debate: “The increased range of motion could not be explained by the structural changes in the muscle-tendon unit, and was likely due to increased stretch tolerance possibly due to adaptations of nociceptive nerve endings.”