Your nerves pull the strings.

As a teacher, often I get students asking me (very discouraged and hopeless), “How can I improve my flexibility.. I cannot even touch my toes…”. I always respond “Just stretch, every day, and it will get better, there is no short cut.” While this is true, and this is how I myself improved my flexibility, I guess for beginners this might sound like a very uninviting task. So I sought to find out more about what really happens when you stretch your muscles.
According to Leslie Kaminoff & Amy Matthews’ book Yoga Anatomy, “in the vast majority of situations, flexibility is not determined by the actual physical length of the muscle or of the muscle fibers that compose that muscle.” In fact, the resting length of the muscles, and the amount it is to lengthen is already set by the proprioceptive nerves endings in the muscle. Proprioceptors (also called mechanoreceptors) are responsible for perceiving the body’s position and movement. Proprioceptors detect changes in one’s physical displacement, and these are found in all nerve endings in joints, muscles. For stretching, these are located in the tendons and muscle fibres. This means it is the nerve endings that are running the show here. It is the nerve endings telling your muscles to what extent the muscle stretch is appropriate, safe and functional.
When a muscle is stretched, the nerves send signals to the spine, this triggers a stretch reflex (or myotatic reflex), which basically involves the muscle resisting the stretch by contracting. This prevents injury, as in cases where muscle overstretch is unintended, there could be risk of muscle tear, so this is a protective reflex of the body.
What happens when you hold a stretch for a prolonged period of time is that the proprioceptor nerves gradually becomes accustomed to this new length and the amount of singaling to the spine (to CONTRACT! and to PROTECT!) is reduced. Thus, with time, the stretch receptors will allow this lengthening of the muscle. In other words, by stretching for prolonged periods of time, you are actually telling your proprioceptor nerves, “This is okay, this is safe, I am not in danger.”
With this, I now can tell my students that stretching is so much more than just uncomfortable and routinised repetition, it is also about conscious attention and awareness, re-conditioning our nervous system to become habituated to deeper stretches. Hopefully, this will bring a little more light and hope to their journey towards greater flexibility.
Kaminoff, L. & Matthews, A. (2011). Yoga Anatomy (2nd Ed.). USA: Human Kinetics.