Disclaimer: Geeky stuff ahead. If you're afraid of physics, do not read. Also, I don't have everything in order just yet, but I wanted to release this before going to Poland, so the missing Figures will be inserted ASAP.
Now Anna, a good friend and student of mine, decided to work on her high kicks. Determined, she told me she was going to stretch a lot. That's the point where, as a trainer, I felt the need to intervene and set things into persepective. You see, while some people might definitely benefit from some targeted flexibility training, what most need is actually mobility. Where's the difference? Well, in a nutshell, flexibility measures the "length" (or rather, the ability to relax) of your muscles, whereas mobility measures the range of motion (ROM) you can take a joint through under control. For my german speaking readers, this is pretty much the difference between "aktive Beweglichkeit" and "passive Beweglichkeit" [WEI2010]. The terminology has been around for years, only the right conclusions aren't always drawn.
Take a simple test. Get into a prone position and lift your legs. Not too far, mind you, perpendicular to the floor is much too steep an angle. Rather, we're talking something like maybe 30 - 45 degree off the floor. Abduct your hips as far as possible, i.e., push your feet towards the floor. Figure 1 illustrates the set up. Now measure (or have someone measure) the distance between your feet. Et vóila, this is your theoretical best kicking height.
Now if that best theoretical kicking height is below what you need to land a high kick, some stretching and flexibility training might be warranted. Grab a copy of [Thomas Kurz] [Stretching Scientifically] or [Pavel]'s [Relax Into Stretch] and get to work.
On the other hand, not everyone is made to kick high. Check out [CONT2015] for further information on hip structure variability and the possible implications. Also note that Brett actually points out that highly flexible hips are more prone to injury, so buyers beware - don't overstretch those hips. You want to develop powerful kicks and stay healthy while doing so, after all, ballet-like hypermobility won't get you anywhere in the long term. Remember: Don't add flexibility to instability.
If your actual kicking height is the same (or more, as you're using momentum to propel that leg up) as the test result, then more power to you, stop reading. This article isn't for you. If, on the other hand, your actual kicking height is below the result of that test, you should be thinking about where that difference comes from. In my opinion, the answer is more often than not quite simply gravity. That's the major difference between a prone flexibility test and a weight-bearing situation. So if gravity is the game-changer, then what is the limiting factor? Strength, I'd say. Namely, hip abductor and lateral core strength.
Think about it. In order to kick high, you need a nice abduction in that kicking hip. However, that abduction is quite limited, so it needs to be combined with an abduction of the supporting hip as well. This will tilt your pelvis in such a way that you need less supporting hip abduction to reach your target. Basically, we're talking about distributing the load over two hips instead of overloading one. There's more to that, such as anatomical limits regarding abduction, so regardless how strong your hips are, there's no two ways about it - you need to tilt that pelvis. Figure 2 illustrates the concept. In the left picture, I'm basically abducting my kicking hip as far as I can without tilting my pelivs (at least, I'm trying to keep my pelvis level, but that's nearly impossible). The red line indicates the pelvis level. Perpendicular to that is the yellow line - basically this is where my leg would be if my pelvis was in neutral position and gravity was acting on the leg (or, conversely, if I was in anatomical neutral and there was no gravity). In other words, this is where the leg would be if I were standing. We use this to measure abduction. Green is the leg's spatial alignment. As you can see, in the left picture, my kicking leg abduction is somewhere near 40 degree, while in the right picture it's about 50 degree. That's roughly a 10 degree difference, which already makes a difference, but might stem from the fact that I was trying to keep my pelvis level in the left picture, so a slightly higher abduction might have been possible. Also, keep in mind that the presented geometry is rather a rough approximation. Despite that 10 degree difference in kicking leg abduction, total difference sums up to roughly 40 degree - most of that comes from the abduction of the supporting hip. That abduction in part comes from muscular strength, but note my torso angle - by intelligently shifting weight, the supporting hip naturally abducts with little effort.
[free body diagram]. That means that we're only considering a discrete snapshot in time. Hence, we're breaking our comparison down to a point where everything else is equal (as in, other than knee flexion angle in the end position). Bottom line here: if your high kicks aren't working the way you want them to, try to actively go through the chamber position and see what happens. Once you got your technique efficiently and flexibility up to par, yet still cannot kick high, it might be time for some general strength training. For starters, this boils down to deadlifts, squats (go deep and heavy on those, but refer to [CONT2015] first) and kettlebell swings. As long as those aren't a part of your training regimen, you're doing it wrong. This is non-negotiable, as Pavel would say. In our Athletics classes, we're always training two out of the following three movement patterns in each session: squat, hip hinge, lunge. Note that I'm saying movement patterns here, not movements: a hinge could be a deadlift, cable pull through, kettlebell swing, broad jump, even a hip thrust. The lunge category would include actual lunges (walking, reverse, side lunges, drop lunges), split squats, step-ups, etc. You see where this is going. Same, but different. Now if you're unsure how picking up heavy weights would benefit your high kicks, consult your anatomy textbook: the gluteal muscles are both your main hip abductors as well as hip extensors. Now strength is angle specific, there's no doubt about that, you'll definitely need specific training as well. Still, everyone's dealing with limited time resources. Before taking up any activity or training measure (think stretching), you have to do a cost-to-benefit consideration. I can almost guarantee you that if general strength training does not currently take an important part in your training (or worse yet, if if never has), that's where cost-to-benefit becomes most favorable. Give it a try, see what happens. If you're already following a well-structured general strength training program where you're actually squatting and deadlifting, yet even still can not kick high well, it's time to incorporate some specific kicking drills. I'd call that specific strength training. Now we can argur all day long if anything other than actually throwing hard kicks is specific strength training, but those drills work, so again, give it a try before getting hung-up in definitions and terminology.
Probably the most simple drill is the wall supported high kick drill. It's straight-forward and requires no partner, so go ahead and practice this one often. Just grab a wall and lift one leg as high as you can, while keeping it chambered. Make a point of pushing your supporting hip forward and tilting your pelvis. From here, just slowly extend and re-champer your knee for a given amount of repetitions. I'm not particularly fond of giving fixed numbers of repetiations, so just set a goal and work up to that goal. If you find yourself compensating by too much lean against the wall, stop the set, take a moment to recover and do the next set with the other leg. You should feel a slight burning sensation on the outside of your hip. If your supporting hip impinges, you're probably not contracting that glute enough - push the hip forward harder and see if that improves things. Some trainers like to progress this exercise with resistance tubes or ankle weights, to place an even higher demand on the hips. Although there's merit to that practice in highly specialized areas of martial arts, where keeping that leg up for a long period of time is necessary (such as semi-contact pointfighting or freaky x-treme martial arts - you bet [Chloe Bruce] does train her abductors more than most of us would), I'm generally not a big fan.
The point is that, when the wall supported high kick drill becomes too easy, your abductors are probably strong enough for a high kick. That means you should practice your kicks by, well, kicking high. Duh. If you can do the wall supported high kick drill, but can't kick high well without a wall, then something else isn't functioning properly and no amount of abductor strengthening will get that in order. The problem might come from your technique (go revisit that again), your core stability or your proprioceptors. The next drill will take care of those.
If I had to find a name for this drill, I'd probably call it the three-count roundhouse kick drill. Good thing I'm not in the business of finding fanc names for drills... You can do this one alone, or wit a partner. I prefer the latter over the former, as it will naturally take care of work-to-rest ratios. In the video above, I present it with Hanna, on of my best high kickers. Hanna is a blue belt and just passed the instructor course. Never mind the TKD outfit - we're going to Poland soon and are not allowed to fight in kickboxing pants and shirts, so the kids are getting accustomed to more traditional clothing.
Basically this drill really just an extension of the wall supported high kick drill. This time, we're targeting not only static strength (i.e., holding that thigh high and playing around with the level arm by extending the knee), but also dynamic (concentric) strength, core control and proprioception. Assume a combat stance opposite to your partner. Lift your lead leg to a chamber position. From here, slowly throw a kick to the belly bottom, followed by a kick to the chest and a kick to the head, all without setting the leg down. Repeat four times for a total of twelve kicks, four at each height. Don't set down the leg before you threw the last kick. After that, your partner throws his twelve kicks without setting his leg down. Both of you repeat the drill for five times and thus, a total of 60 kicks. Then go for the other leg. With that drill, you're training the concentric portion of hip abduction, i.e., lifting your leg from belly level to chest level and from there, to head level. Also, since there's no wall to support you this time, your core will work extra shifts.
The whole thing shouldn't take you more than a couple of minutes. If you do it every day, five days a week, that accumulates to 300 kicks per leg per week. In a month, that's around 1.200 kicks per leg. In a year, it's 15.600 kicks per leg - all in a couple minutes of extra work every day. Do you think that would fix a thing or two in your high kick? You bet it would.
Now again - I'm not against the thai kick by any means. As a matter of fact, I'm even teaching it to some of my students instead of the more TKD-ish snap kick (although in most cases as a first step, to have a functional base to progress from). Still, I'm rather tired of hearing the argument about how the snap kicks lacks KO power. Mirko Cro Cop, anyone?
That's a snap kick. Doesn't lack KO power in my opinion. What do you think? Need more examples? No problem, just look up people such as [Bill Wallace] or [Benny Urquidez]. I'm sure you can come up with a lot of more contemporary examples (did you know [Anderson Silve holds a black belt in TKD]?), but I'm content with the ones I brought up. My part's done for now - the ball is in your court now. Hit the gym, fix your technique, deadlift, practice the drills I gave you and improve that high kick. Be sure to leave feedback if this worked for you.
don't get hurt
References[CONT2015] "No two hips are the same", Brett Contreras
[WEI2010] "Optimales Training", Jürgen Weineck, Spitta Verlag