One-platoon system

The one-platoon system, also known as iron man football, is a platoon system in American football where players play on both offense and defense. It was the result of smaller roster sizes in the early days of the game and rules that limited player substitutions, rules that are also standard procedure in many other sports but were eliminated in the 1940s as free substitution was legalized. The alternative system is known as the "two-platoon system", or simply the "platoon system", because of its use of separate offensive and defensive units (three platoons if special teams is also counted).

Each system was used at different times in American college football and in the National Football League. One-platoon football is seen in modern times mostly on lower-end and smaller teams at the high school and semi-pro levels, where player shortages and talent disparities require it; the system allows teams to play with a smaller roster than a two-platoon or multiple-platoon team, but because players are on the field the entire game with no rest between series, players slow down and become fatigued more quickly in the later stages of a game, hence why it has since become obsolete.

Prior to 1941, virtually all football players saw action on "both sides of the ball," playing in both offensive and defensive roles. From 1941 to 1952, the National Collegiate Athletic Association (NCAA) allowed unlimited substitution. This change was originally made because of the difficulty in fielding highly skilled players during the years of the Second World War, in which many able-bodied college-age men volunteered for or were drafted into military service. The National Football League followed suit abolishing its substitution restrictions in 1943, for similar reasons.

For the 1953 season, the NCAA emplaced a set of new rules requiring the use of the one-platoon system, primarily due to financial reasons. The system allowed only one player to be substituted between plays, which effectively put an end to the use of separate specialized units.Tennessee head coach "General" Robert Neyland praised the change as the end of "chickenshit football".

...
Wikipedia