MMA
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The second reason why is because MMA has limited injuries. It sure is one of the bloodiest sport in the world, but the fighters in the cage are hardly seriously injured. Although some injuries can have the fighters in a shelf for a few months,
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Last is because the sport is pure. There are almost no whining, money mongering, or anything that goes against sportship. Sure, there are people like Conor Mcgregor who thinks he's the best of everything, at the end of the fight, the fighters shake hands and try not to be that guy who does not accept that he/she lost. MMA is just fighting, and it happens to be one of the most natural sport known to man.
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MMA is the best, comprehensive sport ever made. It mainly includes Jiu-Jitsu and kickboxing, but it also includes many other martial arts such as muay thai, and judo, and taekwondo.
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Of course there are some people like Bruce Lee who have been successful with Jeet Kune Do, but you can't think everyone can be good as him with Kung Fu. Now, I do know that MMA has its own rules, but they mostly don't matter, and also you learn how to defend your face and your body in MMA.
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One reason why MMA is the best sport in the world is because it can actually help you protect/defend yourself. Only learning martial arts Kung Fu, Taekwondo, or Karate can be helpful in their own rules, but they can do nothing against a UFC fighter , because they have their own fighting styles that doesn't help you in MMA, which is why people who learn those sports learn other sports as well.
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which probably is basically a serious injury, but they are not like ACL tears or Tommy John surgery from basketball and baseball. It's obvious that some fighters, like Anderson Silva or Frank Mir are capable of breaking a bone or two, but that's why there are empires in the cages.