PHILADELPHIA >> Bill Maas spent 10 seasons in the NFL, most of them as a 3-4 nose tackle. Asked to describe the position once, he said it’s like being a fire hydrant in a dog-peeing contest.

The point the two-time Pro Bowler was trying to make in his own colorful way was that nose tackles take a big-time beating.

Their job is to try to eat up as much space and as many blockers as possible. They sacrifice their bodies so that others can reap the glory of stuffing the ballcarrier or sacking the quarterback.

Because of the punishment they take, their shelf life is short. It has become a young man’s position. Of the 15 teams expected to use the 3-4 as their base defense this season, only four have a starting nose tackle older than 29 — the Colts (33-year-old Aubrayo Franklin), the Packers (33-year-old Ryan Pickett), the Patriots (31-year-old Vince Wilfork) and the Eagles (soon-to-be-32-year-old Isaac Sopoaga).

With the Eagles making the switch from a 4-3 to a 3-4, they needed someone who had NFL nose-tackle experience. Enter Sopoaga, a nine-year veteran who signed a three-year, $11 million deal with the team in March that will pay him $3.75 million this season (a $2.75 million roster bonus and a $1 million base salary).

The former Hawaii standout comes to the Eagles from the 49ers, who had one of the best defenses in the league last season, finishing second in points allowed, third in yards allowed, third in yards allowed per carry, second in first downs allowed and third in third-down percentage.

Sopoaga’s 2012 numbers were unremarkable, but then a nose tackle’s stats usually are. He played only 33 percent of the Niners’ defensive snaps last season — his lowest percentage since ’08 — and had just 55 tackles, one sack and one hurry.

The 55 tackles were his fewest since ’06, which was his third year in the league.

“Isaac’s a veteran,” Eagles defensive line coach Jerry Azzinaro said. “He understands the tempo that we play at and the tempo we practice. He’s doing a really good job for us.”

Sopoaga’s playing time has been progressively shrinking the past three years, from 583 snaps in 2010 (when he primarily played end) to 466 in 2011, to 335 last season. But a lot of that had to do with opposing offenses using more three- and four-wide receiver sets against the Niners and prompting them to counter with nickel and dime packages that don’t include a block-occupying nose tackle.

Despite playing on one of the league’s most dominant defenses, Sopoaga was ranked 145th out of 148 defensive tackles and nose tackles last season by Pro Football Focus. The year before, he was ranked 31st. In his defense, he played much of the season with knee and ankle issues.

The obvious question now that he’s an Eagle is how much of a toll has nine NFL seasons of dog-peeing taken on this Samoan fire hydrant. The Niners 

willingly let him walk, opting to replace him with Ian Williams, a 23-year-old former undrafted free agent who has played in just four NFL games.

He didn’t look good at all in the Eagles’ first preseason game against the Patriots, though few players on Bill Davis’ defense did. He looked a little better last Thursday against Carolina.

“To be honest, I am so blessed,” he said. “I am about to turn 32 (in September) and I feel like my body is carrying a 25-year-old. It all comes from preparing yourself. How well you eat, how well you train. You have to take care of your body.”

At this point, it’s still too early to get a feel for how much Sopoaga is going to be on the field this season. For starters, he’s being pushed by talented third-round pick Bennie Logan. And we still don’t know how often Davis is going to use a three-man front this season because, well, he still doesn’t know.

When asked if he thought Sopoaga might play somewhere close to the 33 percent of the snaps he played last year in San Francisco, Azzinaro said, “I don’t know about that. I don’t know the style or what they were doing at San Francisco.

“I’m sure a lot of that had to do with the nickels and dimes and those kind of situations (the defense played). We’re going to play the game the way it plays out and see what happens. We haven’t predetermined that he’s going to play a certain percentage of snaps.

“If the other team is in two-back and they’re running the ball every snap, I would bet that our nose guard is going to be in the game a lot.”