How do you tell a rookie eager to impress an assistant coach with his heart, hustle and hitting to ease off during a scrimmage, to lay off the pad-popping block, the jarring tackle, the direct hit? You don't, and Greg Hartle, the Redskins' new middle linebacker, is loving every minute of this unexpected contact.

Already in the first three days of practice, his wounded right knee has taken a series of cuts and collisions, scrapes on the turf and full-speed running. There is no swelling, no buildup of fluid and, most important of all, no pain.

All of this is not to say that Hartle has completely recovered from a knee operation he underwent last September to repair torn cartilage and damaged ligaments and tendons he suffered in a routine St. Louis practice session in the second week of the regular season.

"I still haven't taken a direct hit on it," Hartle said today. "But I don't want to make a big issue over the knee. As far as I'm concerned, I've completely recovered. I have no problems with it, and I don't expect to have any problems."

There is a different message coming from the training-camp office of Joe Sullivan, the St. Louis Cardinals' director of operatins and the man who decided not to take a risk on Hartle's knee.

Hartle, the Cardinals' starting middle linebacker in the season opener a year ago, played out his option in 1976. Sullivan declined to match the Redskin offer of a $49,000 contract for him, and allowed Hartle to earn his money in Washington.

"Our medical staff told us it would be very difficult to come back from the kind of operation Greg had," Sullivan said."We thought it was too great a risk. We also felt that Greg was asking for the kind of salary you might pay to an all-pro, not someone who just started to play the position.

"Greg also wanted a guaranteed contract. On this team, nobody has a guaranteed contract. And with his knee being what it was, it was simply too big a gamble. I will say this. He's a fine lad, and a heck of a football player if he's healthy.

"He's a devilish hitter. We saw that right away. He's a tough kid who works hard and wants to succeed. He's also very inexperienced, but one thing George Allen does is teach his defense. He repeats, repreats, repeats and repeats again so you have no excuse for not knowing the defense. But again, it all depends on that knee."

Hartle smiled when he was told what Sullivan said about him. He is a slow-talking, Southerner who scratches his full beard and thoughtfully ponders every question asked of him.

He said he does not want to make it sound as if he is bitter about the Cardinals. But it is obvious there is no love lost between him and the organization.

For one thing, he played for only $16,200 last season, his third in the National Football League, after taking a 10 per cent pay cut for playing out his option.

For another, he believes he should have been starting in the middle long before the Cardinals finally made their move, pushing Mark Arneson to outside linebacker and installing Hartle there last summer.

And, finally, he partially blames the Cardinals' resident meanie, offensive guard Conrad Dobler, for the block that tore yp his knee last fall.

"I'm not going to say it was a cheap shot," Hartle said. "Let's just say it was uncalled for. It was a sweep, and we had a rookie defensive tackle who was supposed to keep Dobler off me. He didn't, and Dobler blocked me at the knees. But in practice, you block high, you don't go for a guy's knees.

"Nah, I don't really hold it against the guy. But I am not looking forward to that first game against the Cardinals. I'm not looking to retaliate, but it's going to be very interesting."

Before that confrontation occurs, however, Hartle still must win a starting position. Harold McLinton, who signed a multiyear contract Monday, will hardly be expected to roll over and concede his job. And Rusty Tillman may also be a factor.

"Greg's going to have a hard time beating Mac out," said Sullivan. "The fact that Harold's played the position for so long gives him a great advantage, especially in George's system. But if anyone can come back from a knee operation and win a job. Hartle is the kid who can do it."

Indeed, Hartle has been fighting - and winning - against the long odds most of his football career.

He was a puny 170-pound defensive end in high school, and wound up attending Newberry College in his home town of Newberry, S.C. He is the first player ever drafted b) the NFL from the 700-student school.

"I was lucky to come to camp the year of the players' strike," he said. "I was a 10th-round draft pick from a little school. But we just had 60 rookies in camp, and when I was able to hold my own with players from the major schools, I figured I'd progressed far enough to play with the veterans, too."

Hartle started the Cardinals' first three games at middle linebacker as a rookie in 1974 while Arneson recovered from a broken hand. Then he played mostly on special teams the rest of the year and the next season as well, until Arneson broke his other hand and missed the final three games of 1976.

"It took me three years to get there in St. Louis and I thought that was too long," Hartle said. "I was dissatisfied and uncomfortable for a year and a half. But I'm not worried about any of that stuff here.

"I just want to go out in the practices and learn everything I can, and then prove myself in the exhibition games. I'm not going to make an issue out of starting. If he (McLinton) is a better athlete, then he'll play in front of me. I know hje's got a big edge on me, but I've been in the same position myself. Harold has, too. We'll just have to wait and see. It's going to be very competitive.