One of the things that makes taekwondo fun is head-high kicking.
Developing head-high kicks requires both strength and flexibility, however.
As head-high kicking is something I wanted very much to be able to do, I put a lot of hard work into my kicks so I could do them head high with relative ease and comfort -- and it was well worth it.
Now I work hard to maintain that ability despite the challenges of aging. Like large muscles, flexibility isn't something you "get" and then have forever. It is a struggle to get flexibility in the first place but it is also an ongoing struggle to keep it. The good news is, it is easier to maintain flexibility than it is to get it in the first place.
Don't think of gaining flexibility as "stretching" muscles but rather as growing and cultivating longer muscles -- something that takes dedication on a daily basis and time. Like weight lifting in which larger muscles don't happen overnight or even in a few weeks, flexibility is something that usually takes several months before results are seen.
To get the best results out of training for flexibility, learn to relax. This is where it can get somewhat tricky: it is hard to relax while you are in pain.
You shouldn't have sharp or intense pains, but the tension you place on muscles to encourage the growth of longer muscles is somewhat uncomfortable. And that pain or discomfort usually tends to make a person tense up so the muscles are pulling in the opposite direction we want them to go.
When you begin to feel that tension of stretching, don't focus on the pain; think about how good you feel after stretching and relax in to the stretch.
In training flexibility in the hamstrings -- the backs of the upper legs -- for higher kicking, we want to relax the hamstrings so the muscles fully extend.
But to really get what we think of as "hamstring flexibility," we must lengthen muscles along our entire "dorsum," the back side of our body, from the base of the rear of our skull all the way to our Achilles tendons at the backside of the heels.
I have seen many martial art students drop their chin toward their chest as they try to kick high which in fact makes it harder on them. When you tuck your chin to your chest, you are lengthening your dorsum and reducing your range of motion.
To gain hamstring flexibility for high kicking, we really need to also stretch the back muscles -- upper, middle and especially lower; the gluteus maximus (or "glutes" -- the rear end); and the calves in addition to the hamstrings. By making small changes in body position you can focus your efforts on these various places.
For example, start with a basic seated hamstring stretch: legs together, feet straight out in front. Point your toes and lean forward keeping your back straight, not curled; take your chest toward your thighs. You should feel the tension in the upper hamstring itself or maybe in the lower back depending on where you are tight -- that is, where your shortest muscles are.
Now, pull the balls of your feet back toward you. You should begin to feel it in your calves and behind the knees.
Next, drop your chin toward your chest -- this should curl your back somewhat -- and you should begin to feel tension in your upper or middle back.
So where should you focus your stretch? Wherever you are feeling the most tension for the most part but eventually all of these places, so vary your positions.
The place where you are most likely to get an injury is the place where your muscles are being stretched beyond their comfortable range of motion, so work on the area where you are "tightest," but don't neglect the other areas.
By lengthening all of the dorsal areas, you are giving yourself additional range of motion overall, reducing tension in the antagonistic muscle groups which resist the motions of high kicking, giving yourself more comfort in high kicking motions, and reducing the chance of injury.