Stretching is fundamental to safe karate practice.
Always warm up your muscles before performing any type of stretch--failing to do so could result in muscle fiber tears or other injury.
A PNF (Proprioceptive Neuromuscular Facilitation) Stretch is valuable in that it helps us stretch the muscles without tearing them which can form scar tissue in the muscle fibers. Greater range of motion is generally found to be achieved through active PNF stretching than by cold and/or static stretching alone. Below is a short video demonstrating the basics of one version of a PNF stretch method--your instructor can teach you others--be safe and enjoy your practice!
THE BASICS OF PNF STRETCHING...
(NOTE: Remember, safety is the key to healthy practice! Never participate in any stretching or other physical activity without consulting a physician, especially if you have any kind of medical condition or injury)