A RUNNING BACK in the National Football League starts out with actuarial evidence pointing to a short career. The average back lasts 3 1/2 years, unless he is something special.

Larry Brown, who averaged 255 carries in his eight seasons with the Redskins, was something special. He often played as light as 193 pound and ran the way Vince Lombardi always wanted -- with "reckless abandon."

Now, at age 32, Larry Brown is paying the price in pain for what he describes as his "refusal to be defeated" on every play. Asked how he is feeling around his battered knees, Brown said:

"Better, if I don't try to play any tennis, do any jogging or fast walking. I can't even keep up with the young women. I can't do some things I used to do. I catch cabs around town because if I walk about three blocks, my knees tighten up.

"About three months ago I was in Miami and was tossing a football around in the sand with some kids. I had a noticeable limp in both legs for three weeks afterward.

"Some of the pain and aggravation I had after I first retired has subsided, but I can't even play golf. I go out for, say, nine holes and have to give up after four or five."

Brown had an operation on his right knee. He had a lot of trouble with his left knee after defensive end Bubba Smith, then with the Baltimore Colts, fell on it.

"After that," Brown said, "the knee would move laterally the wrong way an inch or so. It had no stability."

He said artificial turf had a "great deal" to do with his knee injury in 1972. "The turf caused wear and tear on the knee and caused a lot of aggravation," he said.

As to running the ball maybe 25 times a game, he said, "If you touch the ball you get exposure (to the fans and media) . . . a young kid needs it. If you've got a quarterback who can throw well, there is no need to run that much; if not, you gear the offense to the back, and run him.

"Earlier, I had no problem about running a lot. It was a pleasure to know that they (the coaches and quarterback) have that much confidence in you, especially in adverse situations. It felt comfortable; it was a satisfaction to be the one.

"I heard guys say they tried to protect themselves with the way they ran. I was told by Lombardi that many times players got hurt standing around. In the pros you can't prolong your career by playing at 50 percent of your ability."

Consider, for example, that of the five leading running backs for each of the years 1970 to 1975, only Larry Csonka and Franco Harris still are doing well.

Most of the other 1,000-yard men from those years either have retired -- several before age 30 -- been slowed significantly by the constant pounding or reduced to roles as spot players.

Yet, a list of the top 10 rushers of all time, up to and including the 1978 season, indicates there is room for argument.

Jim Brown lasted nine seasons and O. J. Simpson is in his 11th. Joe Perry endured for 14 seasons. Csonka is in his 12th year, including his time in the World Football League. Only Simpson is a darter and dodger.

MacArthur Lane lasted 11 seasons, until he was 36. Calvin Hill is in his 11th campaign, counting WFL service, at age 32.

Brown averaged 262 carries a season, at 228 pounds Simpson averaged 229 carries before this season. Earl Campbell, 224, is averaging 19 carries a game this season; 202-pounder Walter Payton, 21 a game. Chuck Foreman seems to be fading out in his seventh season, at 29.

Brown never had a serious injury, though no one ever ran harder, in tougher situations. Simpson has said that he likes artificial surfaces because he "bounded" on it and ran faster. His legs have been battered. Yet he doesn't wrap his ankles as all other players are required to do for protection.

Simpson also offered some observations on what it take to have a long and successful career as an NFL back.

"For the top rushers," he said, "the biggest benefit comes from spending the majority of their years with the same offensive linemen. My best years at Buffalo were from 1973 to 1976 and (Coach) Lou Saban had pretty much the same group of guys on the offensive line. Walter Payton has had pretty much the same five guys (interior linemen) the last three years or so. It helps if the coach understands that.

"To be effective, the running back has to carry the ball a lot.Of course, the offense has to be geared to running in that case. You run more traps and dives and you don't get as tired as you do running sweeps.

"Show me a good running team and I'll show you an I-formation team. The runner gets tackled by smaller guys (defensive backs rather then linemen) from that formation. I'd say 25 carries is about right for a top back; I was around that. The running back is usually the best athlete, and don't forget -- that much running eventually will wear down your opponents. a

"I did well on artificial surfaces; I just learned how to run on it . . . like running a golf course. Walter Payton also has learned to. The defense has to play 'upstairs' -- the tacklers can't submarine you or they will get scraped by the surface and injured.

"But once you get hurt, you have a tough time on it. Now that I have had a cartilage removed from one of my knees, there is no spring in it. I used to bounce on artificial surfaces and thus run faster. I stopped wrapping my ankles, like other players do, in 1970, when I had an injured knee; because it restricted my ankle motion and put added pressure on the knee.

"I tried wrapping my ankle last Sunday because it was a muddy day, but it didn't help -- it didn't make any difference, so I'll finish my career without wrappings.

"I think about 25 carries is about right when you're healthy and have a good offensive line; I enjoyed that many carries. I was more effective the more I carried. Fewer carries would be wrong for Payton and Earl Campbell. But if the defensive line is getting penetration, 25 carries is a load."

Asked about Foreman's career in Minnesota apparently being shortened by so many carries and pass receptions, Simpson said, "Foreman has never had the luxury of a good run-blocking line. Fran Tarkenton had the advantage of a good pass-blocking line, but Chuck has been tagged by the big guys for most of his career.

"Even Ed White (the Minnesota standout who was traded to San Diego) was not the type of pulling guard that Foreman needed. Chuck has been discouraged the last two years, I know that. All good running backs are cursed if they don't have above-average lines.

Gale Sayers also lived with painful knees and his career was limited to seven seasons in Chicago by five knee operations. He now is 36 and athletic director at Southern Illinois University.

"Today football is very specialized," he said. "When I played, I also returned kickoffs and punts. Twenty-five carries are not a lot if you are strictly running from scrimmage.

"I can walk around," he said, slighting the impact of his old injuries. "I can play racket ball, but I am sore after two or three tough matches. But I have no complaints.

"I practiced on artificial surfaces and I didn't like it. I played on grass. I first played on a artificial turf in 1958. It changed the game.

"You could stop quicker and run faster on it. O. J. Simpson played on it in college. Today's kids have grown up on it, in high school and colege. They know how to run on it. They have no way of knowing the difference over a career because they didn't play on grass.

"Injuries are caused today by players becoming so much bigger and faster. I started playing football (on grass) in fourth grade. By eighth grade I ran pretty much with the style I later used as a pro.

"My joints started to wear down even in high school, I think. I may be dead wrong, but I think it was from the way I made sudden cuts.

"My coaches in high school and college didn't believe in lifting weight like they do today. My knees were loose already, when I got to the pros. I got hit right and they went.

"The kids are so big today, their joints are wearing down before they get to the pros. Something's got to give.

"When I was at the University of Kansas they wouldn't recruit kids for basketball from New York City because their knees already were dead from playing on the cement in the playgrounds there.

"It's the same in football now, from the kids playing so much of their careers on artificial turf and from being so big . . . so heavy on their legs. When I got to pro ball, it was my time to go, because I had played so long -- from fourth grade up.

"I do not jog now; I ride a bike with my wife. There is no strain on the knees; it even helps build them up."

Wilbert Montgomery of the Eagles said, "Twenty-five carries is not a lot if you expect to rush for 1,000 yards. Most backs don't get started until the second half; but you have to get your rest, about eight hours a night."

His coach, Dick Vermeil, added, "Not to give the ball to Wilbert 25 or more times would be dumb (for a) coach . . . like driving a car in second gear. If we had another Wilbert I'd have each run the ball 15 times. There is a contrast to the past, because most running backs take better care of themselves today."

Coach Paul Brown once was questioned about having Jim Brown carry the ball so often.

"When you have a thoroughbred, you run him," the coach said. "That's what he gets paid for."

Besides the pride in the position, there is another incentive -- the pay. Virtually every back's contract has bonuses; for so many yards gained in a season; for making the Pro Bowl, for helping his team win so many games, make the playoffs and so forth.

And a consistent 1,000 yard rusher usually can be assured a six-figure contract far sooner than many of his teammates.

But then again, some have paid an exorbitant price. And the pay days, for the most part, do not last very long.