Martial arts can demand a lot on a body
Why do these elite fighters spend so much time working out? It's not because they're vain and want to look good. (At least, that's not the main reason.) The fact is, truly elite martial artists and fighters devote a lot of effort and time into becoming, literally, the best athletes on earth. They have to be. During a sparring match or a real fight, the demands placed on their bodies are far greater than those for any other sport.
A fighter needs to have the cardiovascular endurance of a long-distance runner. His heart and lungs have to be able to keep going strong, even after an hour of hard fighting or sparring. But unlike a long-distance runner, she also must be strong, able to lift not just her own bodyweight, but manipulate her opponents' mass over and over again. Most distance runners can't even lift their own suitcases.
A competitive martial artist needs to be as explosive as a sprinter. His every move must be able to explode faster than the eye can see, going from zero to Mach 3 in nothing flat; otherwise, he won't be able to block, parry, or strike effectively. But that explosiveness has to last. A sprinter's pretty much useless after his first 10 to 20 seconds of effort, but a fighter who fades that quickly will be in deep trouble if she's facing someone as good as she is or tougher than he is.
A fighter has to be strong, like a power lifter. As mentioned before, a martial artist - even if he's not a grappling specialist - has to be able to move his body smoothly, quickly, and effortlessly, and force/finesse her opponent into doing what she wants. But that power has to endure. While a great weightlifter or bodybuilder can lift some pretty impressive poundage a few times (say, 10 times max), his strength rapidly fades after that. But a fighter has to have her power and strength last throughout the fight; each and every punch has to have the ability to take her opponent's head off, because he's never sure which one of those punches will connect and have the opportunity to do so. He must be able to oppa just as effectively 45 minutes into a match as he could in the beginning of the fight. (An oppa is a grappling move in which a fighter lying on his back bucks his body in order to displace his opponent, who has mounted his torso and is thus in a position to rain down blows on the head and shoulders from a position of safety.)
