For many people, the front leg kick is a very effective "stopping" move when faced with a charging opponent. The faster (and more accurately) a defender can deploy this kick, the better the opportunity to prepare a counterattack. What exercises have been shown to help develop frontal leg speed such as what you need for a typical front kick (beyond just practicing front leg kicks)?
As my sensei says, "there is the right amount of power and speed in the technique". Basically, what he is saying is that if you do it right, the speed will come on its own.
That said, there is a difference between speed, strength, and power. It's one thing to say you can put 400lbs on your back and squat with it. That speaks to strength. It's another thing to say you can move your foot at 24 ft/s. That speaks to speed. However, putting the two together to move 400lbs off the floor at 24 ft/s tells a whole different story. It's one of power.
There is a reason I wanted to define those terms, and it speaks towards the difference between sport karate and self defense. When you go to the average TKD tournament that does point sparring, most of the contestants train for speed. Just speed. There is no power behind those kicks, nor is there any reason to generate power in a point sparring match. However, speed alone is not going to stop a 250lb man charging you. To do that you need power. Power is generating force over time.
There are a few things you can do to train for power vs. raw strength or speed:
- Heavy bag work. You practice the technique, but the bag has resistance and bad technique is punished. Having all the power in the world does nothing if you don't learn how to kick with the ball of your foot and prevent your ankle from bending. Start light, and increase intensity when form allows.
- Plyometrics. It doesn't require a lot of skill, but box jumps and the like will help you generate more power from your hips to your legs.
- Power Cleans/Olympic style lifts. Much more technical, but allows you to generate a whole lot more power throughout your entire body than either of the two other options listed. Oly lifts include: clean & jerk, and snatches.
Another option is to do your heavy bag work with ankle weights. I do not recommend doing that, but absolutely do not if you can't kick a heavy bag at full force without hurting yourself. The problem with the ankle weights is that it provides unnatural leverages, and can wear your joints out prematurely. If you do choose this route, be smart. Start with light weights, do not fully extend your legs on impact (other wise you will hyper-extend them and injur yourself), and don't ever put on more than 5 lbs. When you take them off, you will be quicker. My sensei did use this approach when he was young, but he also had to have both his hips replaced at a relatively young age.
Stairs work really well. Power up and down, take them multiple steps at a time, go sideways, backwards, everything. Put as much vertical power into your step as possible.
Second thing to try is working in front of a mirror or with a videocamera. This way you can watch your form and see where your sticking points are and where you're making unnecessary moves. Half of the wasted time is probably on the windup for the kick; get that as minimal as possible, and just strike in a direct line.
Thirdly is to walk a lot on the balls of your feet. There's not a really good way around this: you need well developed calves and lower legs for strong, accurate kicks. You may look into
Qinggong, which is all about toning and honing the lower body in a traditional kung fu manner.
And lastly, of course, is to practice kicks. But you probably wanted other advice. :)
Regardless of the physical attributes of speed, your reaction time will largely be determined by how quickly you notice your opponent moving in, and how well your nervous system is trained to react with that technique in that specific situation. No training outside of sparring can give you that kind of natural reaction.
I think reaction time drills might suit you better.
Like Dave said, spar copiously, but with intent.
Work on conditioning yourself to not think, to kick. This is a basic overview of what you are trying to overcome, but its lacking on the practical ways to train it. I will see if I can dig up some actual exercises.
I think crucial to speeding up the whole kick is to be as relaxed as possible for the most part of executing the kick.
When you pull up the knee to your chest, it should be a relaxed, swift movement. Even when pushing forward the leg, apart from the muscles used to propel it forward should stay relaxed, in a way that you would be able to abort the attack at any time. Only in the very last part of the technique you put your weight in the kick by moving the hip forward strongly and put tension on your leg in order to achieve higher impact on your target.