Getting tissues to stay in a stretched position can often require some patience. We have to remember that muscles are composed of Fascia ( a type of connective tissue) and also that the muscle have a component of the nervous system attached to it. In a course that I took, the general philosophy was to hold the tissue in a sub maximal intensity for a longer period of time, often 2 or 3 times as long as you think you would need to.
The general nature of the connective tissue that binds muscles together (and holds them together) are that they are made up of something called the Ground Substance. The great thing about this Ground Substance is that if it is held in a gentle stretched position for a long period of time, and given the chance, it will rearrange itself so that when the tension is released, it will better stay there. I like to use the analogy of stretching leather. If you are stretching leather really hard for a short time, it will probably creep back to where it was originally. If you hold the stretched leather for longer, then it will stay there. It’s the same thing with muscles.
In addition to those properties, there are stretch receptors that monitor the amount of stretch being applied to the muscle tissue. If the muscle has been shortened for a long period of time, then the Central Nervous System assumes that is the normal position and will want to keep it there, especially if you are trying to stretch it. So, the deal is you want to try to stretch the muscle without making it fire too much (which will, if effect counteract what you are trying to do. This is where the sub maximal pressure comes into play. If you hold a stretch at 50 – 60% of what you think, for twice as long, I think you will get to where you need to be, you won’t cause the stretch receptors in the muscles to start firing the muscle that you are stretching and it will much more likely stay in that stretched position giving you much better flexibility.
Be gentle and hold for longer. It’s better.
Look at this good video