Well, you may be surprised to know that a lack of flexibility is not a structural issue. Often, we think of tissue like an elastic band. It seems like a good metaphor, as we'd like to stretch out that band enough to a point that it remains lengthened.
|How we visualize muscle on stretch.|
In other words, when you try to reach for your toes, your hamstrings experience a reflex that cause a contraction against the stretch.
You can't think of muscle shortening or lengthening like an elastic band. You have to think of it like a second person pulling back on a rope when you try to pull it in another direction.
|What's really happening.|
This being said, getting muscle to lengthen and remain like that is not as easy as simple as performing prolonged stretching. If inflexibility is a nervous system reflex in response to stretch, that reflex needs to be trained out. Whether that be teaching the muscle to relax under tension, addressing a reason for excessive muscle guarding, or some other type of conditioning, it always comes back to making the nervous system happy and willing to let go.