What do NFL running backs, Olympic sprinters and top bodybuilders have in common? Wheels of steel. No, we're not talking about an old-school Grandmaster Flash album or a trucking-simulation video game. These athletes all have legs powerful enough to propel them through a brick wall -- in fact, their legs are so hard they look like they're made of brick. Ask any of these athletes: Legs are the foundation of an awesome physique. So where does that leave you? When did you last train your legs? You can admit it -- you skip leg day once in while. While we're on the subject of skipped workouts, what's your cardio status? Yeah, we thought so. Well, here's a concept: combination legs and cardio training. By putting these two dreaded activities together, you get a more intense workout than you could by training them separately and you're less likely to skip either one.