Official football rules limit the number of players allowed on defense to 11. So why does it feel like the Vikings have 20 defenders on the field at one time?

Those guys are everywhere.

They make quarterbacks feel claustrophobic by collapsing their pockets. They swarm ball carriers like bees at a picnic. They don’t want to concede even an inch.

“This is probably one of the most fun defenses I’ve played on,” Brian Robison said.

That description echoed throughout the Vikings locker room Sunday after their defense rendered another offense toothless in a 31-13 win over the Houston Texans at U.S. Bank Stadium.

“Oh man, it’s fun,” cornerback Captain Munnerlyn said.

“Everybody is having fun out there,” nose tackle Linval Joseph said.

Well, not everybody. Not those poor offenses that keep getting chewed up and spit out.

The Vikings increasingly look like a Super Bowl contender in large part because of their defense. That unit is so stingy and overwhelming that anything seems possible.

The Vikings are playing championship-caliber defense, which remains a remarkable transformation compared to their calamity three seasons ago.

The Vikings owned the NFL’s worst defense back then. They couldn’t stop anyone.

Now their defense ranks among the best, if not at the very top. Their defense is a treat to watch, the way all 11 guys work in concert, flying around the field ferociously and unselfishly.

“We’re about one thing and one thing only: winning,” Robison said. “Guys aren’t out for themselves, to pad their stats. They’re not being selfish and putting others in bad situations.”

The Vikings have star players on defense, but they don’t necessarily have one best player. The sum of their parts is a true defining characteristic.

“Just one heartbeat,” cornerback Xavier Rhodes said.

Chemistry can be overused in sports vernacular. The term is hard to describe but easy to identify. The chemistry within the Vikings defense is unmistakable. Everything just fits. Their confidence is boiling over the sides of the pot.

“We enjoy each other on and off the field,” linebacker Anthony Barr said. “That’s the best part of this. We’re very similar in terms of personalities and what we like to do when we’re not playing football.”

They like to do the same things when they’re playing football, too. Mainly, wreak havoc on their opposition, particularly quarterbacks.

The Texans and quarterback Brock Osweiler became their latest roadkill.

The Texans managed only two field goals and a garbage-time touchdown. They didn’t get a first down of their own doing until two minutes left in the first half. Their first three first downs came courtesy of penalties. They converted only one of 13 third-down chances in all.

The combination of Mike Zimmer’s scheme and his defense’s talent continues to make quarterbacks act like a bunch of Nervous Nellie’s. Aaron Rodgers looked rattled. So did Cam Newton. Osweiler played that way, too.

“It seems like they have their scheme mastered,” Osweiler said. “They got after us.”

The Vikings create pre-snap confusion by disguising blitzes and shifting players, making it impossible to know who will rush and who will drop into coverage.

Barr said he noticed Osweiler’s eyes focused on the pass rush at times, rather than looking downfield to his receivers.

The Vikings registered four sacks, 13 quarterback hits and eight pass breakups.

“The defensive line is doing a great job of getting after the quarterback,” Munnerlyn said, “and on the back end we’re covering them up.”

Two plays highlighted their effect on the Texans’ psyche. Osweiler threw an ill-advised pass down the middle of the field in the third quarter. Safety Andrew Sendejo closed fast, lunged and snagged it.

Harrison Smith ran behind Sendejo with his arms stretched out from the opposite direction. He was in position to intercept the pass, too.

The Texans faced fourth-and-16 on their next series. They called a screen pass.

That seemed odd, except it wasn’t because they had no chance to contain the Vikings’ ears-pinned-back pass rush long enough for their receivers to get downfield.

“We feed off each other,” Smith said.

It’s become a feeding frenzy with their defense. They attack from all angles, front to back, side to side. They’re giving the Vikings a decisive edge every time they take the field.

“Right now it’s the best defense I’ve been around,” Joseph said.

Chip Scoggins chip scoggins@startribune.com