I train brazillian Jui Jitsu. I wanted to wear a fitness tracker to figure out how much work I do in a session because they are very intense, the warm-ups are what most fitness places call a 'work-out'. You can wear them in the warm up however the trouble with them is they get torn off when you fight and they *can't* track the amount of work I am doing.
If you train Jui Jitsu, why do you feel the need to quantify the amount of work you're doing? It's like needing a special device to tell you how much fun you're having at a party.
If you need to quantify your martial arts workouts, you're missing the point of martial arts, no? Maybe I'm wrong and am just not hip with the kids these days, who seem to seek numerical validation for everything they do in the form of "likes" and "favs" and "retweets".
In thirty years of martial arts training, study and instruction, it has never occurred to me that knowing the number of calories I'm burning will be in any way enlightening. But then, I'm not really part of Apple's target demographic.