. >> i'm james hobart, 28 years old. my hometown is otis, massachusetts. >> reporter: james is here to show his prowess at the crossfit games. one of the athletes competing in the four-day smackdown in southern california. >> for an event like this, try to think how bad it's going to feel. >> reporter: with 10,000 crossfit gyms around the world, the high-intensity strength training program has become an obsession for millions. it's hard to argue with a body of evidence like this. do you feel like it's become an obsession? >> yeah. >> reporter: for better or worse? >> for better. it hasn't done anything unhealthy to my life. only good things. you know what i mean? obsessive only in the sense that it takes up a lot of my time. >> reporter: the sport has also come under fire. the crossfit fail videos, showcasing the dangers of the court if not done properly. and james is not immune, either. check out that. >> i fell in a hand stand, trying to get the last couple steps. it feels good. i'm a little black and blue. >> reporter:

: 28-year-old james hobart is on a quest to be the fittest man on the planet. >> don't get caught upight. you'd be better off keeping your head down and staying moving. >> reporter: he's competing in 13 grueling events under the harsh sun of southern california. crossfit has become a national sensation. millions of americans are obsessed with the high-intensity strength training program. and it's hard to argue with a body of evidence like this. crossfit is also at the center of a fierce controversy. accused of pushing people too hard too fast. no matter the consequences. the devotees insist these exercisers aren't to blame. >> the program is very safe. when applied properly with coaching and instruction. >> reporter: now, they're competing in the ultimate smackdown, the crossfit games. >> that's tomorrow night. we go inside the crossfit games, where they'll crown the fittest man and woman. make sure to tune in right here on "nightline." thanks for watching abc news. "world news now" is coming up soon with overnight breaking news. woman: so, girls, you're out there, you're selling coo