Most teams in the National Football League use a 4-3 defensive alignment, or four linemen backed up by three linebackers. This versatile defensive front provides coaches options against the pass and run. When zeroing in on run-first quarterbacks, coaches attempt to cover the gaps while prevent sweeping outside runs.
4-3 ISO Strong
Rick Kravitz, writing for the American Football Coaches Association, describes the 4-3 defense as one that can stop the run and prevent big plays by closing gaps on the defensive lines. Kravitz wrote, "Close all inside seams and spill everything to the outside where we can run it down." In the ISO Strong, the middle linebacker fills the main gap and knocks down the fullback before he cross the line of scrimmage. The strong-side and weak-side linebackers close off their gaps in the line to force the quarterback to commit to a direction. This allows the defensive backfield to cut off the runner and end the play.
4-3 ISO Weak
Kravitz writes that in the 4-3, the linebackers are in control of stopping the play. In the ISO Weak, the weak-side linebacker hits the fullback two yards deep in the backfield and takes him out of the play. At the same time, the nose guard has drawn a double-team, creating a mismatch on the line of scrimmage. This permits the middle and strong-side linebackers to cut off the quarterback's running lane.
Base 5-2 Defense
The Base 5-2 is an old-style defense that includes five defensive linemen: a nose tackle flanked on each side by a defensive tackle and end, and two linebackers. Joe Daniel, a football coach and author, writes at his website Football Defense that the 5-2 depends on having a strong nose guard who can take the center out of the blocking scheme and control the main gap. This allows the remaining tackles and ends to fill the line and close off the remaining gaps, allowing the linebackers to work the corners and make the tackle.
4-3 and the Option
High schools and colleges employ option offenses to confuse defenses with the run. With a quarterback and two running backs that can take a pitch at any second, the option often presents problems for defenses. In the 4-3, Daniel writes that the middle linebacker focuses on the fullback, who can decoy, block or carry the back. The defensive end, guarding the "c-gap," or outside gap in the line, picks up the quarterback. If the quarterback pitches, the linebacking corps and defensive backs are available to swarm the ball carrier and stop the play.
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