JKD would win against TKD or Capoeria!
TKD depends on maintaining distance with its oppoent. Its kicking distance techniques are used to attack and maintain the right distance. Kicking is a form of defense in a way. TKD is strongest in long distance fighting, therefore in order to defeat TKD, you just have to move in and fight in close distance where the kicks are useless. JKD could easily do this because it has good counter-attack theories and techniques. It was based on the best counter-attack martial art - Wing Chun. JKD could counter the kicks of TKD and attack during the most unexpected moment with close distance offensive techniques, e.g. punches. TKD would not be able to defend against the fast and powerful punches of JKD. JKD also has the one-inch punch which was made famous by Bruce Lee. It's a technique where the user can punch with full strength in an inch or less. It was also in the movie Kill Bill 2.
Capoeria would also lose against JKD since it's techniques aren't very powerful or fast. JKD would just keep attacking and Capoeria wouldn't be able to do much. JKD is much attacks faster and it's defense is also better. Capoeria wasn't even a actual martial art. It used to be dance used to entertain people. It wasn't really used for fighting.
TKD is more of a sport. There are rules that limit TKD's full abilities. This gives TKD a disadvantage and a boundary. Capoeria is more of a dance, but JKD is based on street fighting. It's a theory of real-life fighting where there are no rules. It's not a martial art, although many people think it is - it's just a theory of Bruce Lee.
I agree that the results depend a lot on the actual martial artist fighting, but I think JKD has the overall advantages and this affects the results a lot.
Jet Li is Shaolin Kung fu. He used to be a Shaolin monk and he became a movie star after he won the Chinese Wushu Chamionship when he was a kid.
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