Defending a running back from criticism has been as simple as blaming defenses for "stacking the box" against him.

It's usually not that simple.

The subject came up here most recently after we considered why the St. Louis Rams had so few "explosive" running plays last season.

'12 Rushing vs. Loaded Box, 1st/2nd down

Perhaps defenses weren't respecting the Rams' passing game, allowing them to load up against the run. The offensive line could share in the blame. Game situations could have been a factor. And it's always possible Steven Jackson was declining with age.

ESPN's Matt Williamson, writing the appropriately named "Eight in the Box" item for the NFC South blog, noted that Jackson might fare better with the Atlanta Falcons because "he will not be facing stacked boxes down after down" as he had with the Rams.

The subject is begging for analysis, but first we need some information.

Jackson ranked fifth in 2012 rushing yardage against loaded boxes on first and second downs. He averaged 4.3 yards per carry on these runs, a respectable average.

The Seattle Seahawks' Marshawn Lynch led the NFL in total carries against loaded boxes. He ranked second in rushing yards gained in these situations.

The chart at right shows the top 12 players in rushing yards against loaded boxes, according to ESPN Stats & Information.

The chart below ranks 2012 offenses by how frequently they faced loaded-box situations on first and second downs. I removed third and fourth downs to avoid obvious passing situations and most short-yardage situations. The idea was to isolate straightforward running plays. The figures do include quarterback rushes -- nine plays for Seattle, but not a significant number for most teams.

ESPN charts box counts by looking at the "number of defenders at the snap who are within five yards of the line of scrimmage and no further than two yards outside the tackles or outermost player attached to the line" -- excluding defenders following skill players in motion unless those skill players come to a stop while the defender remains in the box. Defenders just outside the box area who are clearly rushing into the box are counted as in the box.

Got that? I'll wait while you read it again.

I'd love to wrap up this item with an air-tight conclusion, but I'm not sure there are guiding principles applying to all teams equally. Different defenses likely have different reasons for loading up against specific opponents in certain situations, with disparate results.

Seattle's opponents stacked the box on 20.1 percent of rushing attempts, the highest rate in the NFL. There could have been other times when Seattle changed plays to a pass in these situations. That could happen more frequently in the future as quarterback Russell Wilson gains experience.

Defenses could have less reason to stack the box against pass-oriented teams with quarterbacks able to exploit numbers advantages down the field. I would have expected the San Francisco 49ers to have faced loaded boxes a higher percentage of the time. However, their frequent use of heavier personnel could have invited a higher number of defenders into the box area without producing a "loaded" situation -- one reason not to assume the frequently referenced "eight in the box" represents a numbers advantage for the defense in every case.

Perhaps you've got some theories. Fire away.

2013 Rushing vs. Loaded Fronts on First, Second Downs