After the Colts gave up 257 yards rushing against the Houston Texans last Sunday, a lot of questions have been asked. One of the biggest questions is, how could this complete defensive breakdown occur?
One of the primary responsibilities of the defensive front seven, which includes the Colts four defensive linemen and the linebackers, is stopping an opponents running game. Every team handles this responsibility differently but in the Colts system, which uses smaller defenders than any other team in the league, there is a specific process the defenders must follow to achieve their 
The Colts defense uses a one-gap defensive philosophy which require the front seven defenders to maintain control over each running lane available to potential rushers. There are two gaps outside of the opponent’s center, two outside of the guards, and two outside of the offensive tackles. Technically two additional gaps are created in two-tight end sets, outside of the tight ends. Each defensive lineman is responsible for “filling” one of the gaps, and the linebackers are expected to fill the gaps that remain.
Generally, on each down one linebacker and one cornerback or safety is given an additional responsibility as the primary and secondary containment defenders. Their role is to stop the rusher from getting outside of the defense, forcing the rusher in to where defensive traffic is heavy and away from the sidelines. The linebacker’s job is made easier or more difficult by the success or failure of the defensive linemen.
If defensive linemen fail to fill their gap, and a running lane is created in their gap, the linebackers either have two gaps to fill, or a far wider gap than the defense intended. If defensive linemen are able to penetrate their gaps, the offensive line will often respond by redirecting one of their blockers to the spot along the line that is failing, in an effort to stop defenders from potentially re-routing or “blowing up” the running play. Once this breakdown starts, the linebackers are given cleaner lanes to fill, can more easily read and react to the ball carrier, and will generally use their speed to stop running plays for short gains, and sometimes losses.
Once running backs are able to shoot through an open running lane, they get to the “second level” of the Colts defense, which leaves the linebackers to handle running backs in the open one-on-one and typically a lead blocker as well. The lead blocker, in particular, makes the linebacker’s job difficult because the player has to find a way around or through the blocker to reach the ball carrier.
In games like the one against Jacksonville in 2006 and against Houston in Week One of the 2010 season, these responsibilities regularly broke down, and led to statistically embarrassing run defense, and sound losses. The defensive line was regularly blown off of the line, away from the ball, gaps were left unfilled, linebackers failed to fill those gaps, and too often containment failed.
The best way to fix the problem is for defensive linemen to have better success filling their gaps, and linebackers to not over-pursue and do a better job getting off of blocks.
Against Houston, the linebackers too often ran directly into blocks, and seemed too eager to look for help from players behind them to make the play. Relying on safeties and cornerbacks to stop rushers regularly is not a recipe for success. If the players on the field are incapable of filling their assigned gaps, the coaching staff will need to find substitutes who can do that job.
With Antonio Johnson limited against the Texans on Sunday, and with no other legitimate run stuffing defensive lineman on the depth chart behind him, the Colts suffered and their weaknesses were exposed.
The only other way to make up for this lack of run stopping depth on defense is to control the game offensively. If the Colts play with a lead, it becomes far more difficult for opponents to rely on the run, and the focus of offensive formations and blockers will change in such a way that stopping the run becomes easier.
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