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?