What happens when you stretch

This article discussed the importance of stretching and how the muscles are affected when they are stretched. Stretching is vital to the health of muscles because when the muscles stretch, the muscles lengthen. The two types of muscle fibers are intrafusal muscle fibers and extrafusal muscle fibers. Intrafusal muscle fibers lie parallel the the extrafusal muscle fibers, which are the ones that contain myofibrils. Proprioceptors are nerve endings that relay information about the muscles to the central nervous system. They are used in stretching to send messages about the muscles lengthening too much or too suddenly. The stretch reflex is the reflex where the muscle attempts to resist muscle lengthening by contracting. Over time this reflex can be slowly reduced by a continuation of stretching it so that the signal is ignored by the body. Dynamic stretching relates to movements that only have sudden increases in muscle lengths, while static stretching involves long lengths of the muscle being stretched. Nuclear chain fibers are responsible for static stretching, while nuclear bag fibers are responsible for the dynamic portion. When one muscle contracts, it causes another muscle to relax and lengthen. This is known as reciprocal inhibition. 

"The proprioceptors detect any changes in physical displacement (movement or position) and any changes in tension, or force, within the body."

I found this quote really interesting because I never knew about proprioceptors before. I had always assumed that there were just average neurons and nerves in the body that sense these things. Do malfunctions of proprioceptors contribute to muscle cramps or twitches?

"This triggers the stretch reflex (also called the myotatic reflex) which attempts to resist the change in muscle length by causing the stretched muscle to contract. The more sudden the change in muscle length, the stronger the muscle contractions will be" 

I found this interesting as well because I have experienced this myself many times. This reflex happens when I try to stretch, showing me that I am not flexible and have to go slower. It helps me realize my limits.

"When this tension exceeds a certain threshold, it triggers the lengthening reaction which inhibits the muscles from contracting and causes them to relax" 

I still find this phenomenon really interesting. I also experienced this first hand with the hamstring stretch and saw how much farther my leg went after flexing my quadricep. How can the line between healthy and excessive be drawn for stretching?