Mixed Martial Arts, or MMA, is extremely challenging and fun!!! It not only pushes your body to it's limits but challenges your emotional and mental prowess as well. Unlike other martial arts, MMA is more susceptible to causing frustration to any competitor. You not only worry about your opponents strikes from the hands, knees, elbows and feet but you have to be conscious of their ground, or grappling skills as well.
I find martial arts a great challenge for anyone. Combat aspect of the art awes me because it is a dangerous yet compelling sport to be a part of and you have to have your wits about you.
Don't forget to stretch, though, injuries can be brutal.