I used to do bjj and muay thai for a little over a year. I gained confidence and strength. But I had to leave because my fitness goals were higher. My buddy who stayed, and practices very regularly has very low bf% but no traps or much of any discernable muscular definition. If I would advise an absolute beginner on what sport to start with, I'd say some martial arts.
But also one has to consider how martial is your "martial" art. Many sports have been effectively neutered and are a joke. Ehm karate, king fu, krav mega(never was), etc. Or things like cardio boxing where you're more dancing than anything else.
I loved jiu jitsu because I pushed myself to discomfort constantly and I rolled 100% most days, you can't do that with many martial arts because that would mean impact to the face and head, and that's not good on a regular basis. Muay thai was intense, sparring was tough, but I couldn't take the constant bashing of shins. It's a hell of a way to fight, very painful to the opponent and you(potentially).
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