The NFL's All-Ironman team

Despite playing over 1,000 snaps in 2013, Dontari Poe graded out positively across the board. Scott Cunningham/Getty Images

In the early days of pro football, the great ironmen of the NFL played both offense and defense, and practically never left the field. These days, the league is too specialized for that; you play either offense or defense, not both. Rarely does a player find snaps on both sides of the ball, and never for more than a handful of gimmick plays, such as the Arizona Cardinals trying to get Patrick Peterson some time on offense.

Today's NFL ironmen are something else: They are guys who stay on the field in an era of constant substitutions.

I'm not going to say it's easy to make an impact if you're a situational player, but it's easier if you know you can give 100 percent on every snap without fear of getting fatigued. For players that know they will be out there every down, being able to maintain a dominant level of play is a special trait, one which we are going to recognize here with the Ironman Index.

Let's look at some of the league's current ironmen -- guys who play the majority of their team's snaps while performing at a consistently high level -- using last year's snap count data: