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The following popular article on stretching claims that lack of flexibility is purely neurological (or even psychological) and that [almost] any unconscious person would be very stretchy, e.g. can be put into a full horizontal split:

When unconscious, we are all capable of full splits, yet when awake our bodies sense impending danger to the muscles lengthening beyond this preconceived point and begin to tense up to prevent injury. That stiffness you feel when you stretch is all in your head and totally created by you.

-- Andrew Read, You Can Already Do the Splits: How to Relax Into Stretch (emphasis mine)

I have heard similar claims before. It sounds very implausible but should be easy to test. Is it true or false?

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amoeba
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    Perhaps this would be a better fit on [fitness.se]? – Reinstate Monica -- notmaynard Sep 15 '15 at 21:20
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    The claim itself seems to me to be ambiguous: it says "our bodies sense impending danger to the muscles" - so is it actually dangerous to do this? In which case, what's the claim? Our unconscious bodies can presumably be put into all kinds of dangerous and harmful positions. That doesn't mean the reluctance to be placed into them is an illusion. – AmbroseChapel Sep 19 '15 at 06:38
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    I guess a drunk person is the next closest thing to "unconscious". And those are easier to find, than fainted ones. – Vorac Mar 13 '17 at 16:33
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    I learned (undergraduate in Biology and Sports, ca. 1993) that this "preconceived point" at which the muscles tense up is *physiologically* triggered. I.e., there are elements (the name of which escapes me at the moment) embedded in the muscle that detect overextension, and trigger a contraction. That's not "all in your head". – DevSolar Jan 02 '18 at 10:09
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    Found it. The (English) term is [muscle spindle](https://en.wikipedia.org/wiki/Muscle_spindle), which are related to the [stretch reflex](https://en.wikipedia.org/wiki/Stretch_reflex), which -- by the definition of "reflex" -- is not "wired" to the brain (as that would take too long, rendering the reflex ineffective -- these buggers take a shortcut that goes no further than your spinal cord ;-) ). – DevSolar Jan 02 '18 at 14:32
  • @DevSolar That's right, the "is all in your head" part of the above quote is clearly nonsense. That said, whether the "When unconscious, we are all capable of full splits" assertion is true, depends on how the state of unconsciousness affects spinal reflexes. See https://biology.stackexchange.com/questions/38807 that I asked (and later answered) around the same time in 2015. – amoeba Jan 02 '18 at 14:41
  • From the same studies I also remember that maximum flexibility can be trained up to a certain age (around 20?), and after that all you can achieve is keeping it (somewhat) from deteriorating. I also saw a coed (who came from a vaulting background) do "stretching" exercises that went *well* beyond anything I have ever seen anybody do outside of a circus tent; and that's me as former martial artist who easily out-limbered most of my peers at stretching but never came even near to a full splits. (She also tore tendons twice in one year...) That's why I doubt the claim, without hard sources. – DevSolar Jan 02 '18 at 14:48
  • (ctd.) Meaning to say, if you never trained for flexibility, I find it unlikely that your muscles, tendons and joints will have the *physical* range for a full splits. Never tried it the two times I had somebody lying before me unconscious, I was busy with other things at the time, like establishing recovery position... -- Also, anaesthesia is a bit more than "just" unconsciousness. – DevSolar Jan 02 '18 at 14:53

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No. The claim, that "when unconscious, we are all capable of full splits", is not supported by evidence.

There is not a great deal of literature on this topic. One paper on a similar topic “Neurophysiologic influences on hamstring flexibility: a pilot study” concluded:

  • Averaged over all 11 subjects, no change in the flexibility of the hamstring was measured as a result of anaesthesia,
  • For the 3 patients who had spinal anaesthesia, there was a small increase (of only 8.1 degrees) in flexibility during anaesthesia,
  • Mean popliteal angles with hip held at 90 degrees were around 130 degrees, meaning, the subjects were not particularly flexible, either before or during anaesthesia.

So, while the study implicates some neural contributions to muscle flexibility, it is likely not the only one. Also, the fact, that it is possible to increase one's flexibility seemingly instantly eg. by taking advantage of the Golgi tendon reflex, suggests, that a neural component exists.

But: the hypothesis behind the claim, that the only thing stopping us having perfect flexibility until we get to the physical limitations of the joints themselves, is our neural control, is inaccurate and not supported by evidence based on clinical experience with anaesthesia.

See a longer discussion of the topic here.

BKE
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    Thank a lot for answering this old question. Happy to accept it. However, see this Q that I posted and then eventually self-answered on biology.SE back in 2015: https://biology.stackexchange.com/questions/38807. The title question is "Do spinal cord reflexes (such as the knee-jerk reflex) continue to function under general anaesthesia?" and the short answer is "Yes". So it's theoretically possible that the only limitation is neural even if flexibility does not substantially increase under general anaesthesia. I guess spinal anaesthesia provides additional insight though. – amoeba Jan 04 '18 at 22:39
  • Thanks, the answers on the biology SE question are really interesting! – BKE Jan 04 '18 at 23:00
  • I'd like to check if the spinal anaesthetic used by Krabak et al (paper you referenced) is known to block monosynaptic reflexes, but I cannot access Krabak et al at the moment. Will try from work later. If you have access, can you check what drug was used? – amoeba Jan 04 '18 at 23:05
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    [Found it](http://journals.lww.com.sci-hub.tw/cjsportsmed/Abstract/2001/10000/Neurophysiologic_Influences_on_Hamstring.6.aspx). They used "spinal anesthesia with 0.75% bupivacaine". I could not quickly find a direct confirmation that spinal bupivacaine blocks monosynaptic reflexes, but I would assume that it should. It's definitely interesting that there was 8 degrees increase in the spinal group and basically 0 degrees in all other groups. – amoeba Jan 04 '18 at 23:19
  • An 8 degree instant, temporary increase would also be reachable for conscious persons with some protocol making use of the inverse myotatic reflex. It is interesting, that the study seems to suggest, that it is the maximum achievable in the short term. It would be nice to see more studies on this. – BKE Jan 05 '18 at 00:29
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The study ( “Neurophysiologic influences on hamstring flexibility: a pilot study” ) mentioned above clearly says:

"Overall, the intraoperative angle was significantly greater than the postoperative angle"

This result implies that the anesthesia did in fact cause greater flexibility in the hamstring during surgery.

Michael Bud
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    "significant" has a different meaning in science. It means "measurable". So while in common parlance we would describe a large increase in flexibility such as doing the splits to be "very significant", a scientific paper's use of that word does not imply a large effect. Hence, a statistically significant increase in flexibility does not imply a positive answer to this question's claim. – piojo Dec 01 '19 at 05:01