When stretching in an exercise we must learn to listen to our body and only stretch to as far as is comfortable. To many people who exercise think it is only beneficial to us if we make it painful and give it 105% of our effort. The body has a safety mechanism that comes into play when we put our body in a state of pain this is the “Stretch Reflex” which contracts the muscle there by not letting the muscle stretch. Constantly putting any muscle into the “pain” zone will not stretch it more but just break it. The only way to stretch a muscle is to relax and listen to it and only go as far as you can feel slight tension usually 75% of where you could go if really pushed, ease of then stretch again. Building slowly up to stretching more and more. Little and often is always best in exercise.
This also can be used in our daily work, if you constantly go at every thing 105% you will eventually break.