It used to be the teams with the top player taking snaps from center had an edge in winning the Super Bowl. Last year, the Ravens showed the big defensive tackles who could destroy those guys behind center were the difference makers.

	Opposing blockers can't push Tony Siragusa around, which makes running on the Ravens a weighty proposition.

That's what is so funny about the past offseason. Instead of copying Brian Billick's passing schemes, teams are scrambling to match the defensive skills of Sam Adams and Tony Siragusa. The Bears signed massive Ted Washington and Keith Traylor to add bulk to their defense. The Seahawks used Chad Eaton to anchor their revamped defensive line and John Randle to disrupt blocking schemes. Six teams drafted defensive tackles in the first round.

"Ray Rhodes is doing the same thing in Denver, getting Chester McGlockton and Leon Lett together with Trevor Pryce," Ravens general manager Ozzie Newsome said. "That's why Jacksonville drafted Marcus Stroud. Cleveland and Chicago did it, too. By getting those big tackles, you force everything east-to-west and you let your fast guys (the linebackers) get them. No one can get into a power running game against you."

That's what the Ravens and others have been building for years, and the trend is forcing major changes in the game. Don't discount what Tony Dungy has done in Tampa Bay because teams such as the Jets, Rams, Cowboys and others are copying the Bucs' concept of a powerful front line, quick linebackers and defensive backs who drop into two-deep coverages.

The formula is the same, though. The Bucs crowd the middle of the blocking scheme with Warren Sapp and Booger McFarland. They may not be as big as Adams and Siragusa, but they are just as disruptive. They force the action to a quick set of linebackers that have made those old, slow tackle-to-tackle middle linebackers out of date.

Ray Lewis, Sam Cowart, Brian Urlacher and others with speed and tenacity are becoming the dominating play-makers because the big tackles are keeping the centers and guards from blocking them and giving them the freedom to chase ball carriers and turn them into victims.

"You want play-makers in your defense," Lewis said. "But you can almost contain an outside linebacker by going to one side of the field away from that outside guy. It's no fun when you are playing the weakside of the defense and they run away from you. It's better to be in the middle where you can run sideline to sideline and stay on the field all the time."

The Ravens have it all. Their cornerbacks -- Duane Starks and Chris McAlister -- are physical and can cover and are among the best in football. Safeties Rod Woodson and Corey Harris have speed and smarts. The linebackers are fast. But the key is what happens up front. Adams, with his quick first step, can get to the quarterback by his third step in retreat. And two blockers can't move Siragusa.

"The game has changed," Ravens defensive coordinator Marvin Lewis said. "You have athletic quarterbacks and you have big running backs. For the big receivers, you have to counter that with guys on defense who can move. What you can't have is that old run-up-the-middle-stuff-this-guy middle linebacker."

That's affected the Jessie Tuggles, George Koonces and others.

	“	You have to be able to stop the run ... If you can't stop the run, you have to commit more people to stop it and it becomes a vicious cycle. ”
		— Mike Holmgren, Seahawks coach

"Think about the evolution of time and how the college game has changed, too," Lewis continued. "You're taking defensive ends and turning them into linebackers. You are taking safeties and turning some into linebackers."

But speed at linebacker and in the secondary doesn't work if the defensive line can't win battles. Once the Eagles drafted defensive tackle Corey Simon and the Saints signed run-stopping defensive tackle Norman Hand, they jumped into the top 10 teams on defense.

"You have to be able to stop the run, and we couldn't stop the run last year," said a humbled Mike Holmgren, the Seahawks' head coach. "Statistically, we were in the bottom of the league on defense. If you can't stop the run, you have to commit more people to stop it and it becomes a vicious cycle. We knew we had to fix it up the middle. If you have two defensive tackles and a middle linebacker, you've got a chance to stop the run."

How important is stopping the run? Of the top 10 teams in run defense, seven made the playoffs and the top two -- the Ravens and Giants -- were in the Super Bowl. The Colts were the only playoff team among the 10 worst run defenses, and they were a first-round casualty to the Dolphins.

The Colts, despite not having a lot of cap room for defensive acquisitions, hope to upgrade those numbers with the additions of defensive tackles Christian Peter and Mike Wells, a 325-pound run-stopper with a 550-pound bench press.

It also shouldn't be a surprise that the NFL has turned into a defensive league, and it goes beyond the transition from the Dan Marinos, John Elways, Jim Kellys, Troy Aikmans and other traditional quarterbacks. Seventeen head coaches are former defensive coordinators or special-team coaches with defensive mentalities.

"Defensive coaches believe running the ball helps the defenses, and you are very comfortable if you get a good running game," Cardinals coach Dave McGinnis said.

Which is why an NFL record 23 teams had 1,000-yard backs. Of the eight who didn't have a 1,000-yard runner, the only team that had a winning record was the Eagles, whose 1,000-yard caliber runner Duce Staley was injured but quarterback Donovan McNabb ran them into the top 15 in that category. Those other seven non-1,000-yard teams won a combined 34 games, an average of about five.

"Most teams, if they are patient, can run the ball on anybody," Cowboys coach Dave Campo said. "Defenses can't play in seven-man fronts anymore. If you play with seven guys committed to the run, teams can get 120 or 130 yards in the game. But you've got to discourage teams from running the ball."

So if the defensive tackles put a huge detour sign around the middle, you have a chance. Of the teams that made such changes, the Seahawks and Browns have the best chance to show dramatic improvement. The Rams will also be tougher to run against with the addition of Damione Lewis, but they still have to prove they have a pass rush. The Bears have similar concerns.

"When I was at Washington last year, we ranked second in defense before Norv Turner got fired because we had an all-star team on the defensive line: Dana Stubblefield and Big Daddy (Dan) Wilkinson at tackle, Bruce Smith and Marco Coleman at ends," said Browns defensive coordinator Foge Fazio. "Gerard Warren adds bulk and enthusiasm to the Browns' line."

The enthusiasm comes from not being run over by the league's top backs.

John Clayton is a senior NFL writer for ESPN.com.