I do not believe in static stretching.
In one of Pavels books he mentions that while most people can not perform a full split, they can still bend one leg 90 degrees sideways. In another question: Instantly increasing flexibility by becoming angry as a result of pain I mention some indications that adrenaline make you both stronger (hysterical strength) and more flexible. It seems to do this by "taking the brakes off". I am also influenced by this: https://www.jtsstrength.com/what-is-tightness-and-why-stretching-isnt-the-answer/?v=c2f3f489a005. I therefore believe that static stretching do not cause any structural changes in the muscles or tendons. Instead it is a purely neurological effect. By doing a stretch repeatedly you are instructing your nervous system that this position is safe. I believe that the nervous system have an estimate of the strength of the muscles and tendons. It measures the tension via the Golgi tendon organ. If it exceeds safe value it disallows movement. By stretching you are instructing it to be less careful. This can potentially be harmful.
So when one stretch one is telling the nervous system that it is safe to allow a larger range of motion (ROM). This has to be repeated daily over months for the nervous system to take notice.
But is it safe to allow a larger ROM? I for instance can only do about a 90 degrees split as opposed to a full 180 degrees split. And with good reason; I am heavy and the deeper I go down the stronger the forces on my groin musculature becomes. My nervous system protects me from muscle tears by restricting my ROM.
So I believe that lack of flexibility is in fact really always a lack of strength. If I had really strong groin musculature my nervous system would allow me to do a full split.
More specifically I believe that the problem is lack of strength in lengthened state. Take a look at strength as a function of muscle length:
The problem here is the steep downhill on the right side. As an example say that my 90 degrees split is halfway down this hill. My nervous system will not let me lengthen my muscles any more than this because I am too weak in that position.
Bodybuilders sometimes work on their biceps peak by doing restricted ROM curls. This tells us that it is possible to change the shape of this curve (to some extent at least).
So I think that in order to become more flexible one should work on strength in lengthened state.
Am I wrong?
If not: how do one best do that?
This article: Eccentric Training for Flexibility provides some scientific references that seems to back up my assumptions and suggest that eccentric training may be the solution.
But how do I perform eccentric training in practice? Since I will be working in the lengthened state where the muscles are weak I conclude that I must use light resistance. I would also think that the exercises should be performed slowly.
Some exercises like the cossack squat seems to be a mix of light strength training and stretching. I would think such exercises are the solution since they strengthen the muscles trough a full range of motion?
Mark Rippetoe seems to hold the position that the solution for better mobility for squatting is, drumrolls, more squatting. In his response to "Squat Depth Issues - The Chicken Or The Egg?" he recommends to "let the weight provide the stretch" which sounds like eccentric excercise to me.