NFL teams have increasingly added a third receiver at the expense of a fullback in recent seasons.

The San Francisco 49ers have stuck with an old-school, power-oriented offense under first-year coach Jim Harbaugh. Old-school is not code for rudimentary or simplistic, however. The 49ers deploy an impressive number of running plays from a long list of formations and personnel groupings, a Harbaugh hallmark when the coach was at Stanford as well.

2011 Personnel: 3+ WR

I've put together a few charts showing the 49ers' and NFC West teams' offensive personnel use by down. The charts exclude fourth-down plays for the sake of clarity.

The first column of the first chart shows the personnel in the most basic form, by the number of running backs and tight ends on the field. Defenses increasingly must factor for variations within these core groupings. But when defenses set their personnel initially, they do so based on these general parameters.

Tight ends Vernon Davis and Delanie Walker are versatile enough for the 49ers to use two tight ends a high percentage of the time. We've seen the team use Walker effectively on "wham" blocks to free running back Frank Gore for long gains, notably against Ndamukong Suh and the Detroit Lions two weeks ago.

2011 San Francisco 49ers Offensive Personnel by Down

The next chart focuses on the use of two running backs together for NFC West teams across first, second and third downs.

We see the St. Louis Rams using the smallest percentage of two-back groupings, as expected when Josh McDaniels became offensive coordinator.

The 49ers are heavier on these groupings even though veteran fullback Moran Norris has missed all but two games. The team has used backup Bruce Miller quite a bit, and Miller has improved. He has played in six games, starting three.

2011 Personnel: Two-back Offense

The next chart shows how frequently NFC West teams have used two tight ends across downs.

The Seattle Seahawks' injury situation at the position has prevented them from using more of these groupings. The Rams are quite heavy on two-tight end personnel across early downs, while the Arizona Cardinals' use them quite a bit on third down.

The Cardinals' tight ends have been more productive as receivers on first down, however. Receiver Early Doucet has 223 of the team's 403 receiving yards on third down, far more than even Larry Fitzgerald (60) and tight ends Rob Housler (31), Todd Heap (28) and Jeff King (7).

2011 Personnel: Two Tight Ends

The final chart singles out one of the heaviest personnel groupings teams use regularly, those with two backs and two tight ends.

Again, the 49ers' versatility at tight end allows them to use this grouping without sacrificing the ability to pass. The Seahawks' injuries at tight end and fullback have made this grouping impractical. The team's improved talent at wide receiver has also commanded more use of those groupings with three or more wideouts.

Game situations also come into play. The 49ers have protected leads more frequently than other teams in the division. Heavier personnel lends itself to the running game.

2011 Personnel: Two backs, two tight ends