Mae Geri, or front kick, is one of the 4 basic kicks in karate. In fact, it's so theoretically simple, I can best sum it up as pick your foot up and extend it out.
SIMPLE! So simple it's brilliant. So simple, that it boggles the mind how so many people don't execute it in an efficient manner. Before we get into how to correct this technique, let's look at it's uses and it's importance.
The mae geri can be used offensively, defensively and as a "stage 2" building block for more advanced kicks. Offensively, a well executed front kick can be used to inflict pain on your target. This application is the most prominent, as it is the most obvious and the application most often taught. It can also be used to push or create space between you and your target, the latter of which is also it's primary defensive application. Defensively, it can also be used to slow or halt the momentum of an assailant/opponent. Progressively speaking, the mae geri is the building block to the nidan geri (double jump front kick), kakato otoshi (ax kick), ko geri (crescent kick) and kake geri (hook kick).
With all of the potential applications of this kick, it's easy to see why being correct AND efficient in execution is important.
Lift the knee in front of the body as high as possible; height is paramount. Extend your leg, from the knee.
Simultaneously, point the ball of the foot at your target; this is your striking area. Ensure your toes are pulled back. DO NOT POINT your toes...unless you enjoy hobbling and have zero intention of wearing open-toed shoes. After striking, retract your leg by bending at the knee. Set your foot down back in starting position.
...or simply stated: pick up your foot, extend your leg, bend your leg, put your foot down. But I'm sure you understood that.
Again, it's simple...right. Right? What's the problem? Let's start at the top...er rather, the center.
During one of my first classes as an instructor, I noticed that almost half the class was kicking mid-shin level. I counted and coached and encouraged, I finally stopped everyone and said "WE DON'T KICK PUPPIES IN THIS DOJO!"; it was good for a laugh, but they'd all understood. However, understanding and physical limitation sometimes meet at a crossroads...and physical limitation usually wins out. I tried and I tried to give tips to get their kicks up, with not much success.
Eventually, I had them sit into full squat position; the students who were having the issue could not fully sit into this position and the students who were kicking just fine could...we hung out here for awhile.
Following standard M-Wod protocol; I had the class stand and try the kick again; lo and behold...the kicks were closer to belt level --almost auto-magically! From that point on, we integrated this stretch (and a few others) into the class warm up and the kick height got progressively better.
Big ups to Kelly "K-Star" Starrett and MobilityWod...I'll be referring to his teachings quite a bit, so you'll be QUITE familiar with him by the end.
Quads to knees:
I have also often times seen students with knee problems have trouble executing this kick. To me, this is an indication of tightness in the quads and hamstrings. As K-Star would say; you need to create slack. The couch stretch is a stretch to open the high hip area. A modified runners stretch will assist with loosening up the high hamstring area.
Point the toes:
Generally, executing a mae geri flatfooted is incorrect. There is some usefulness, however, only in one perceivable instance: stopping your target. For every other use of this kick, you will need to strike with the ball of your foot. In addition to striking considerations, you'll want to get those toes back (trust me).
Let's experiment: walk up to the closest wall and take off your shoe; ram your toes into the wall. Doesn't feel good, does it? That's what's eventually going to happen if you don't get your toes back. One tip that works fairly well is to pull your heel off of the ground before you kick, effectively, setting your foot position before you even move. Another is to walk around on your tippee-toes. Also, consider stretching the calf; it is possible that you lack the flexibility to position your foot.
Let's go back uptown and revisit the hips. One habit that I notice students sometimes tend to not pick up is thrusting the hips into the kick. The hips are a source of power in all techniques in karate; the mae geri is no different.
Try this: Stand next to a wall, take off your shoe. I promise, you aren't going to slam your toes into it this time. Position your foot into a good mae geri position on the wall, with your leg fully extended. Now, thrust your hips forward.
When you stand up and dust yourself off, think about that extra umph added to your kick full speed.
Practice practically: Grab a kick shield and find a partner. Have your partner hold the kick the shield...now kick it. Reset, then kick it again; this time thrust your hips into the kick. See the difference? Now repeat, until your partner is at the other end of the dojo. TUrn and come back.
Lastly, make sure when you execute this technique that you do not roll your shoulders forward; kinesthetically speaking, when your shoulders move forward, your hips travel backwards; when the shoulder moves backward, the hips push forward. Any guesses on what should happen with your shoulders? ;-)
One final point: earlier in this post, I said that height was paramount. Knee height affords you, the kicker, options on where to place your kick. You can't kick head high if your knee is barely waist high, unless you're double jointed --but that's just weird (nothing against people who are double jointed!). Sometimes it is advantageous to kick with a lower knee height, but you don't want to train that way; always train for the optimal application and the situation that will be the majority application. In this case, you'll want that knee nice and high.
There's lots of information there; I think all of it is relevant and useful, though it will take time, effort and practice. That said, with time, effort and practice you can take one of the most basic karate kicks and form it to fit a variety of situations and applications.
Remember: drill, drill drill! Work on your basics with pad & partners, not just in the air. You WILL notice a difference.
Feel free to ask questions in comments if I need to clarify anything (or just to say "hey, good post ;) )