Here's the full article by Martin Koban
The central nervous system, muscle length and the lie of stretching
There are over 200 bones and more than 600 muscles in the human body. Some muscles can be voluntarily controlled; others are used without us contributing much. When it comes to movement however, we have to learn everything from scratch. So when we were born, we already had the hardware to move around, but were missing parts of the software. That basically means the central nervous system needed to learn how to control muscle contractions and how to coordinate them to create controlled movement. This stuff is stored for later recall, since it would be a pain in the ass to have to learn everything again the next morning.
Now think about this for a moment here: the CNS learns how to control and coordinate hundreds of muscles at the same time! The amount of computing power, bandwidth and storage capacity needed for that is unparalleled even by the most sophisticated computer. However, there’s a small caveat. The CNS will adapt your body to whatever you do for longer periods of time.
The lie of stretching
One of the ways this adaptation works is by stopping the muscles from extending beyond a certain point. Pavel Tsatsouline illustrates this important point in his book “Relax into Stretch” with the following example: extend one leg to the side, putting it on a bar stool or table, to create a 90° angle between your legs. Now try the same with your other leg. This should be possible without much of a problem. However, if you now try to spread both legs at the same time to attempt the “Russian Split”, you won’t be successful (and if you were successful, let’s just assume for the sake of argument that you weren’t). Why is that?
Tsatsouline proceeds to point out that “no muscles run from one leg to the other. No tendons, no ligaments, nothing but skin”. The reason why you can’t extend (or rather: abduct) both legs at the same time to do a Russian split is because your central nervous system doesn’t want you to. The position you’re trying to put yourself in is perceived as dangerous by the CNS, so it preemptively stops you from going there. This is the so called “stretch reflex”. It is most certainly not because your muscles are too short.
The central nervous system has gathered information about what is safe and desired for all your life. If you have been doing splits all your life, your CNS won’t have a problem with you going into these extreme ranges of motion and the stretch reflex won’t kick in. If you have been sitting at your desk most of your life and your main form of exercise was using some machines at the gym, your CNS might even stop you from touching your toes and splits are most certainly off limits.
In order to get your flexibility back, you have to retrain your CNS. You have to move through slowly increasing ranges of motion and prove to your CNS that they are safe, so that it will relax your muscles.
(edited by CF)