My problem is that: My thigh lifts up high when I am nearly executing the kick but as I execute my kick, my thigh stays about my waist. What was that cause from? Also I can do middle split and side kick. My major in Taekwondo is poomsae. Sometimes I could kick my leg up high but I could not hold my kick for about 2 second when it reach its maximum height.
Overall, my coach said that your leg could give a good sparring kick but you could not give a good poomsae kick.
Front kicks (and similar teep kicks for those in Muay Thai, Dutch Kickboxing or general MMA striking) take an incredible amount of technique, flexibility, balance, and coordination.
It sounds like your problem is a combo of flexibility and technique. I know you say you can do the splits. That's great, but each kick has its own flexibility requirements. You need to push your flexibility specifically while doing that kick. The only way to get better at it is to drill it more. Stretching out your hamstrings may help a little, but if the technique isn't sound, no amount of flexibility will make up for it.
I mentioned stretching, but another little exercise you can do between drilling the kick is static holds. Lift your leg to kick height and hold it there. Get that knee up high. For 10 seconds. 20 seconds. 30 seconds, etc. It's going to suck. You're going to get tired. Now when you're good at that, throw the kick (under control) from the static hold position. This will build the conditioning for that specific kick, so when it comes time to throw it in real speed your leg will quickly snap up.
All in all, no single thing will fix this issue. Drill the kick more, do your hamstring stretches, and build up your kick specific conditioning!
It is very hard to help troubleshoot without seeing a video of your kicks. You could have a range of problems, and so, without playing 20 questions with you, I'll give you a list of things that are common to people who have kicking problems as you describe. It's probably better for the community anyway. Understand that what you describe often happens to new students, so likely, it is just your body getting used to all of the motions required to execute the technique.
Many of these tips can help correct many different kinds of problems with many kinds of kicks.
Lift the knee
What we strive for is lifting the knee, but danger can set in. Ideally, we want the knee as far into the shoulder pocket as possible, and then the foot whips up. This is dangerous because the technique relies on the momentum of the foot to whip it upwards. If you are not flexible, you can easily pull or tear a muscle. So start light and get an idea of your capability, then work so the knee goes ever higher.
Hip Strength
When I ask a student to throw a kick, I sometimes ask them to hold the kick as long as they can. When they throw the kick as high as they can, but the foot immediately drops a few inches, it means they're more flexible than they are strong. Momentum got the foot up there, but with lack of strength, they can't hold it there. If this is your case, working on abductors and lower back is key. You can keep practicing the kick full speed, or you can do it using dynamic tension, or you can have a partner help you apply resistance (isometric / PNF). These exercises will not only build flexibility, but they will also build strength. And strength is important because it helps you control your kicks. Without control, you are more liable to injure yourself (because of a pulled muscle) or a partner.
Where are your hands?
If you drop your hands to the side as you try to kick, that can sap a lot of momentum from lifting the knee, and that can tend to keep the knee down.
What are you looking at?
If you keep your chin down during the kick, that can help create the tendency to keep the knee down as well. You want your chin forward - not upward and not downward.
Where are your shoulders?
If you hunch your shoulders, than can also create the tendency to keep the knee down.
Pivot on the standing foot
It isn't always necessary to pivot on the standing foot (indeed, there are times you don't want to). But by pivoting, even just a little, you can open the hips just a little more. This can help allow the kicking knee to rise a little higher. Buy not pivoting, a tight hip can tend to keep the knee from rising as high as it could.
You not only need flexibility but muscle strength. In order to practice this, you can do following exercises:
These exercises will help develop the muscles of your upper thigh which are needed to help keep your front kick for a few seconds.
For poomse it is important to show the kick and hold it in position for at least a second. It was hard for me at first too, but eventually you'll get the hang of if. I used to practice by repeating the kick I had to do. For me it was mainly the front kick because its not that easy to keep the leg up front in perfect balance, but honestly just keep practicing and if that doesn't work sparring is not a bad option. Sparring is more less technique and more speed. If you have speed than I suggest sparring because its less power and more speed in sparring. poomse is more technical and involves everything you know and all the power you have to make it look neat and tough. hope that helped!
