What are the most effective ways to increase flexibility for karate kicks?
I am a huge fan of proprioceptive neuromuscular facilitation (PNF) stretching. It is what I used to be able to do the splits cold.
Ideally you need a partner for it, but most of it can be done by yourself although you will have slower or limited results doing it solo.
The premise of the stretch is to relax the target muscles, then force them to tighten and do work, then relax them again - going deeper into the stretch.
There are a couple of points to note though:
you don't need to be massively flexible. Due to anatomical or degenerative issues many people will never be able to do the splits. That's okay - being able to kick people in the head is nice, but not necessary - many of the old schools never kicked above the waist anyway.
even if you do become super flexible you still need to practice kicking technique. Flexibility will simply help you with reach and height and speed, none of which are any good if your technique is still sloppy.
PNF stretching is heavy duty, you need to allow recovery time between stretching sessions. Stretching damages the muscles so you need to be careful not to overwork yourself - twice a week is ample for this type of stretching.
once you do the splits you need to maintain it. If you don't your muscles will gradually tighten and you will progressively lose your flexibility. If you get lazy you will lose it :)
Here is a reasonable Youtube example I've posted before, you can follow the Youtube trail to find more examples.
In my own experience, dynamic stretching is really effective for kicking flexibility. For kicking, you need a dynamic motion in your limbs and joints, so that is what you practice.
For example, rest a hand against the wall for balance, and do leg swings. Keep your leg straight, relax your muscles and lift it up until you feel a stretch, for a number of repetitions. When you begin to feel fatigued (around 10-15 reps), stop and do the other leg or a different exercise. If you do karate, this is very similar to mae keage geri, but the difference is that you should not push it as high as you can from the first go, but only as far as you feel comfortable with a light stretch.
This exercise is done similarly to the sides and back. I also do roundhouse kicks while balancing myself against the wall, with the same set of mind – a steady pace, until my leg tires (usually 16-20 reps).
An added bonus for these exercises, is that you do something related to your class – kicks (but perhaps slightly different techniques). While you perform them, you don't just increase your range of motion, but also your balance, strength and possibly technique (depending on how you'll do the stretches).
Note that the goal here is not static flexibility, but range of motion and warming up the joints and muscles, which is why they should be done in the warmup, or immediately after.
It will also be a good idea to do static stretching, like front splits and side splits, but do it after class. This is because you are extending your muscles beyond their maximum stretch. This will not only tire it and damage it slightly, but it may lead to injuries if you jump up and do dynamic motions right after. Static stretching however, when done right, will increase your maximum flexibility, so should not be overlooked.
I highly recommend reading "Stretching Scientifically" by Thomas Kurz. It explains in great detail not only how to stretch, but also why specific stretches are beneficial for which athletes, and most importantly how to avoid injuries.
PNF is great! I started using it about 10 years ago and it has done wonders for me and some students. It is tough to show this method to students in class to most as there are different skill levels, and sometimes a new student or two.
I try to incorporate this into my karate program as well as yoga poses in the beginning and at the end. I also use straps and a ballet bar and it works wonders too.
Catering to adults over 40 flexibility is key, in karate or just for overall health and wellness; prevent injuries and improved the range of motion.
Just as stated above, in many traditional styles of karate, such as goju ryu karate, head kicks aren't performed due to the close range fighting techniques, but if you can do it well you will definitely hurt your opponent.
And, yes, if you get lazy and don't maintain your flexibility you will lose it; I'm guilt of that, however, over the past 3 months regained much of it through my own classes.