A variant of the I formation used by an offense. In an I formation, the fullback and tailback are lined up directly behind the quarterback, with the tight end on the line of scrimmage and a wide receiver to each side.
In a strong I, the fullback lines up out of the straight line in the direction of the tight end. This gives the team a large number of potential blockers on one side of the formation. The strong I is considered a power running formation for off-tackle and sweep plays, or other running plays that attempt to overwhelm one side of the defense.
The strong I is one of the more common power running formations in football today. The goal is to get an advantage in numbers on one side of the defense in an attempt to open a hole for the running back. Since the fullback lines up closer to the edge of the line, he is in a better position to get a block on a player outside the tackle box to seal off a lane for his tailback. The disadvantage to the strong I is that it leaves less blockers on the back side to seal off quicker players attempting to race through and disrupt the play.