In
American football, there are usually at least two sub-teams of 11 players on each side: the
offense and the
defense. The defensive players and the offensive players take turns being on the field, depending on which side has the ball. This allows them to
rest and conserve their
energy for quick, explosive plays when it's their turn.
Sometimes a player will stay on the field and play for both the offensive team and the defensive team. Such a player is said to be playing iron man because of the incredible endurance needed for such a feat.
The term iron man may also be applied to any athletic event which demands unusual endurance. An ordinary marathon is not specifically an iron man event, because all marathons are equally brutal. A double marathon would certainly be an iron man event.