Roundhouse kick
A roundhouse kick (also known as Round Kick or Turning kick) is a kick in which the attacker swings his or her leg in a circular motion, striking with the front of the leg or foot. This type of kick is utilized in many different martial arts and is popular in both non-contact and full-contact martial arts competitions. Variations exist for low, medium and high height.
In Popular Culture
Roundhouse kicks were introduced to the West by the spread of Eastern martial arts. Possibly due to the move's combination of motion and power--the attacker spins fully around, which makes for a powerful-looking attack--it became a prerequisite feature in many fighting video games (either as a combination attack or a finishing move) and a common 'finishing move' in martial arts sequences in film and television. The latter case is best exemplified in Walker: Texas Ranger, in which the lead character (played by Chuck Norris) almost always defeated the episode's villain with a roundhouse kick, inevitably to the head, shown twice from different angles.
The Walker: Texas Ranger connection is the reason why the roundhouse kick is a common aspect of the "Chuck Norris Facts" internet phenomenon.
Tae Kwon Do Method
The taekwondo roundhouse kick is performed by first drawing the knee straight up in a "chamber" position. This chamber, identical to the chamber of many taekwondo kicks (front kick, side kick, axe kick, etc.) is utilized so that the opponent cannot guess which kick will be thrown (this differentiates it from muay thai, karate, and other roundhouse kicks, which tend to incorporate rotation before or during the rising of the knee). The knee is then rotated so that it is nearly parallel to the ground (counter clockwise for the right leg roundhouse) and the kicking hip is simultaniously rotated towards the opponent. The rotation of the hip, combined with the snapping of the leg forward, gives the kick its power. The striking surface is the instep of the foot.
This is also called an "off the line" or "rear leg" roundhouse kick.
A similar kick is the front leg roundhouse, or "fast kick." To begin the front leg roundhouse kick, the leading leg is drawn up vertically, then rotated and snapped towards the opponent in the manner described above. The fast kick version is done by skipping forward with the rear leg, moving the kicker towards the opponent while simultaniously chambering and snapping the front leg roundhouse. The front kick is generally weaker than the rear leg roundhouse because the hip does not rotate as far; however, it is also generally faster because the leg travels a shorter distance before striking the opponent.
Muay Thai Method
There are several traits which give the Muay Thai Roundhouse a very different feel and look. The main methodological difference is that the person delivering the kick will raise up on the ball of his foot while he kicks, mainly to allow for greater pivoting/turning speed, and increased power (this method obviously has it's detractors, as not all martial artists agree that the ball of your foot is where you should be in a roundhouse kick). The practical difference is the objective of the kick. A standard karate or tae kwon do roundhouse kick has you snap your instep at your objective, but a Muay Thai Roundhouse often uses the combined momentum of the snap and your leg's speed to offer a more powerful and fast kick (albeit at the cost of control, in the opinion of some). Because the kick is executed with much more speed/power, the kicking leg will continue all the way around until it reaches it's starting position.