Tony Dorsett and John Riggins, the two top rushers in the National Conference of the NFL, are about as different in playing styles and roles as two running backs can be.

At 5-foot-11, 190, Dorsett's forte is explosiveness. Speed and shiftiness are his major assets. He is the Dallas Cowboys' game-breaker.

Riggins, at 6-2, 230, is called on to get most of the tough, straight-ahead yards for the Washington Redskins. Speed and strength are his major assets.

Riggins is the meat and potatoes of the Redskins' attack, whereas Dorsett is caviar to the Cowboys.

Dorsett, who gained 1,007 yards and was named NFL offensive rookie of the year last season for the Super Bowl champion Cowboys, was expected to do even better this year and is off to a fast start. He has rushed for 450 yards on 79 carries to lead the conference.

Riggins, basically a blocking back the past two seasons, was written off as a premier runner by many observers.

He gained only 203 yards in five games last year before a knee injury sidelined him. Two years ago, in his first season with the Redskins, he gained 572 yards.

This season, Riggins is right behind Dorsett with 358 yards on 74 carries.

In his first two seasons as a Redskin, Riggins had one 100-yard rushing day. He has already had two this season and appears on the way toward his career best of 1,005 yards, established with the New York Jets in 1975.

While Dorsett's role has not changed since the day he became a Cowboy, Riggins' role was rewritten when Jack Pardee replaced George Allen as the Redskin coach.

In short, Riggins became a running back again.

"This offense has given him a chance to run," said quarterback Joe Theismann. "It's no fun being a blocker all the time when you're supposed to be a running back. John has great speed for a man his size and he is a fine receiver. To not use his assets in the past was just foolish."

Riggins says his role hasn't changed much this season. "My blocking assignments are the same as they were last year, but they're added a few more running plays for the fullback," he said," and they're given me the ball more.

"A big part of gaining yards is getting you rhythm and you have to carry the ball a lot to get into it." Riggins said. "I can't think of too many backs in the NFL who don't want to carry the ball. Quarterbacks want to throw it, receivers want to catch it and backs want to run it. That's what makes the position fun."

"But it is impossible for one man to carry ball every time," Riggins added. "You have to do the other things, too, and I think everyone on this team has shown tha they will do those things."

Even though Riggins is in his eighth season as an NFL running back, he is still trying to improve his running technique.

"George Dickson (the Redskin running back coach) and I have worked a lot after practice on some things," Riggins said. "I have a tendency to run straight up too much because I'm looking for someplace to run. I don't barrel in with my head down.

"There are some times, though, when I need to try to get my weight over my legs more and get under the tacklers."

Riggins' blockers say he is a pleasure to work for.

"When you have a back like Riggins, you have to give him the ball," said tackle Terry Hermeling. "The more he gets the ball, the better he gets. He just seems to get stronger as the game goes on.

"We figure if we get him through the line of scrimmage, he'll make a lot of yards on his own. If we get Mike through, he'll fake one defender out and get more yards. John will run over one. It also helps in that both of them block well."

Pardee said that Dorsett may be the more explosive of the two, "but Riggins is the one we want to get the ball to. Dorsett can break one from anywhere on the field, but Riggins has three or four long runs of over 20 yards. "Anytime you can get 20 yards on a run against the defenses of today, you have to have some explosiveness."

Pardee said wide receiver Danny Buggs, who sustained a slight shoulder separation against the Jets, "will be out indefinitely." He was the only player the Redskins reported to the league office as injured.

The Cowboys said tight end Jay Saldi is out with a broken arm, Tom Henderson and Robert Steele are questionable with sprained ankles and John Fitzgerald. Cliff Harris, Tony Hill, D. D. Lewis, Mark Washington, Preston Pearson and Harvey Martin are all probable with various bumps and bruises . . . . The Redskins brought running back Steve Powell, who was cut by Buffalo, in for a tryout yesterday . . . Pardee said that the Redskins had "agreed under contract to return to Carlisle. Pa., for training camp next season. We'll be there at least through 1979 and probably in the future after that," he said.