Nightly News | March 16, 2013
>>> as countless americans hit the gym or go for a casual run this weekend, for some the obsession with fitness has moved to a whole new level. they've become weekend warriors , challenging themselves in what can best be called a survival of the fittest . we sent nbc's kristin dalgren to find out what it's all about.
>> let's go, let's go!
>> reporter: it used to be a marathon was the ultimate way to push your body to the limit. this is not your father's fitness.
>> everybody together!
>> it's getting back to the roots of playing? the mud. being a kid again.
>> are you ready?
>> reporter: just outside phoenix, arizona, thousands lined up then dragged, climbed, crawled, and fought their way to the finish of what's called a spartan race. one of the latest crazes in extreme fitness.
>> come on, come on, up the hill.
>> we've talked to people who have done marathons, triathlons, they couldn't do one of these.
>> reporter: kobe call has done more than 30 races. he quit his job to become a professional racer. the challenges can be intimidating. they challenge you on things you don't normally do. but it's not just about the physical. you need to be mentally tough to do this. so, why do something so hard, something where you sign a waiver that literally says there is a real possibility you may die? why, says divorced mother of three ann reynolds, is this moment of accomplishment.
>> a lot of camaraderie, a lot of adventure, something always new around the corner.
>> reporter: spartan race is just one of the options for americans who seem to be obsessed with pushing the athletic envelope. in a tough mudder racer run through live wires. and plunge into icy pools. s.e.a.l. fit mimics elite s.e.a.l. training. zombie races, yes, i said zombie races, add in a fear factor. by the explosive growth of all of these, it's become crystal clear . for many, when it comes to working out, ordinary --
>> feels great!
>> reporter: just doesn't cut it anymore. kristin dalgren, nbc news, phoenix.