Learn naeryo chagi: straight leg raised high, heel driven straight down. Build-up via mobility and pad, typical mistakes and safety for head-height targets.
The honest answer: the axe kick is an impressive but delicate kick, and the learning order matters more than speed. You raise the straight leg high above your head and let the heel drop straight down onto a shoulder, head or pad. It is a classic in taekwondo and demonstrations, but a kick that demands a lot of flexibility and control.
Why the caution? In a lab study with a dummy head and taekwondo headgear, an axe kick variant ("clench") reached a head-injury value (HIC15) of about 163, while a boxer's uppercut in a second study was about 24; taekwondo kicks overall showed larger values than boxing punches [1]. That is a lab value on a dummy, but it shows that a kick to a person's head is no game. A review of 88 studies found that kick impact forces are enough to break bones [2].
Realistically you need months of regular stretching and practice until the kick works without swing or pain (experience, not a study). This guide does not replace supervised training.
Another reason to build in stages: the axe kick is popular for demonstrations and videos, where height counts. In training something else counts, namely control at the end of the movement. If you can lift the leg high but cannot brake, you are not practising a kick but a risk.
Slowly raise the straight leg to chest height and hold it for three seconds without tipping the hip. Milestone: five repetitions per leg, the torso stays upright.
Raise the leg step by step, only as far as it goes without pain. No forced stretching. Milestone: you reach your height with a soft knee and a stable standing leg.
Let the leg drop in an arc and brake just before the pad without hitting it. Milestone: five repetitions where the heel stops calmly in front of the pad.
The coach holds the pad at shoulder height. You hit with the heel, the leg stays straight, back and torso stay stable. Milestone: five hits in a row without leaning back.
After contact you place the leg down under control and go into your stance. Milestone: you land without a lunge.
This kick is borderline: it is useful in training on pads, but a head-height target on a partner is risky. A review links the rotation of the support leg to injury risk: pivoted stances increase rotational load on the support-leg knee, and the choice of striking surface decides whether the shin or the foot is loaded [3]. In the axe kick the standing leg carries the load while the other leg is high up.
At World Championships the share of head and neck injuries fell alongside rule changes while upper-extremity injuries rose [4]. Rules therefore change what injuries look like.
Never practise the axe kick on a partner's head. Only pads and bags held by the coach. No use in free play. This guide does not replace supervised training.
Never stretch by force or with pressure from a partner. Stretching with pain is not progress. If one side is clearly stiffer, train that side more often, but not longer (practice standard).
Start the kick from a small step forward, so the standing leg is closer to the target and the high leg has less distance to fall. That lowers the height you need without cheating on flexibility. This is a practice rule; there is no study on it.
Naeryo chagi means axe kick: the straight leg goes up and drops with the heel straight down. There are variants such as bakkat naeryo and an naeryo that you learn with a coach.
You must be able to raise the straight leg clearly above hip height without tipping the hip. How high exactly depends on your goal. Build flexibility over weeks.
For the target: yes, if it hits the head. The lab values on a dummy head are high [1]. For you: yes, if you kick without warm-up or control.
Raising and lowering without a target, yes, with a mat and space. Hitting you practise on a pad with a coach.
As a rule of thumb short, regular sessions suffice, for example ten minutes four to five times a week instead of one long stretching session. Pain is a stop signal. There is no study on this here.
Huryeo Chagi in Taekwondo: The Hook Kick Step by Step