Flexibility is crucial for good form and to avoid injuries.
Flexibility in the hip and the legs will help to kick high without injury. But its not easy to be flexible as we age.
Below are some links to websites that deals with stretching and flexibility:
A detail explanation on stretching:
Stretching and Flexibility - How to Stretchby Brad Appleton
Another website on stretching for kicks:
Flexibility and Stretching by Tim Hall
To help on pain by massage.. useful info
http://saveyourself.ca/tutorials/trigger-points.php
Another detailed useful info site on stretching
http://www.ifafitness.com/stretch/index.html#SEC1