Calvin Muhammad, running like crazy to meet Joe Theismann's pass, dove for a football today and caught a helmet in the ribs instead.

For one minute, then two, he knelt on the sideline with his head held low. He couldn't move, he said later, because of the pain.

Four trainers ran to him. Coach Joe Gibbs turned his head away from his offense to watch something he really didn't want to see.

Then, just when you thought another Washington Redskins receiver had gone down, Muhammad's side finally stopped hurting. He jumped up, dashing back to the huddle, well but wary.

A third wide receiver, if that becomes his job description, rarely has seemed as important to a team as Calvin Muhammad is to the Redskins.

With the return of prodigal son Charlie Brown, Muhammad officially is two-thirds of a starter in the Redskins' training camp.

Brown -- who missed the May minicamp in a dispute over his role with the team -- and Art Monk, added together, are the other 1 1/3. It's the Redskins' new math.

When they lined up this morning, Brown and Monk were running one-two, with Muhammad and Mark McGrath three-four. That doesn't mean the lineup will look that way in Dallas 48 days hence, but it might.

"I didn't make an effort to go in initially with the first huddle," Muhammad said. "I would like to go with the first team, of course. But it really doesn't matter to me."

What means the most to him right now is immersing himself in an offensive system he was thrown into, hands first, last fall. The quickest of studies, he caught 42 passes for 729 yards and became an instant highlight-film star with the Redskins' longest play of the season, an 80-yard touchdown against Dallas at RFK Stadium.

He started eight of the last nine games of the season when Brown was out because of injuries to his hamstring, ankle, fibula and knee, and led the Redskins with a 17.4-yard average per reception. Only Monk, who caught more passes (106) than any other receiver in NFL history, had more receptions on the team.

Muhammad is unlike most 26-year-old pros who have played this game for five seasons. He is talked about as if he were a rookie. Time, the coaches and players say. Give him time. Then watch him go.

"He needs a full training camp under his belt," says receivers coach Charley Taylor.

"As spectacular a year as Calvin had last year, when Charlie Brown got hurt, we definitely lost a little bit," says Theismann. "Calvin and I have to get on the same page as far as our sixth sense goes, knowing where he's going to be.

"Things that Art and Charlie and I have worked out are something that Calvin has to work on, still."

So, he probably should be forgiven for getting angry with the energetic rookie who plowed into him as he dove for a ball off his fingertips. Muhammad didn't get his number, but he might recognize the helmet if he runs into it again.

"Maybe he was trying to impress the coaches," Muhammad said. "I didn't think we were going to indulge in that kind of hard hitting this early. But it was partly my fault; I missed the football."

A year ago in training camp, before Muhammad was traded to the Redskins for a fourth-round pick in the 1985 draft, Los Angeles Raiders linebacker Matt Millen hit him from behind and broke a bone in his shoulder.

"That's the Raiders," Muhammad said later.

Muhammad missed that training camp and was available when the Redskins called, needing someone to replace Brown.

"When I was injured at camp, (Raiders owner) Al Davis told me, 'You'll get your chance.' It just wasn't going to be there."

Muhammad first caught the Redskins' attention when he scored two touchdowns in a 37-35 Raiders victory at RFK in October 1983.

Today, on a field where careers, not games, are won and lost, he makes his mark with his feet, quick enough to earn him an invitation to the 1976 U.S. Olympic trials.

"Watch him, running down the field," Taylor said. "Every play, he goes 70, 80 yards. Just running. He loves to run. And it catches on. Charlie gets down the field, Art gets down the field . . . "

Muhammad, who ran the 100 meters in 10.2 seconds but skipped the Olympic trials to get ready for his freshman year as a football player at Texas Southern, says what he loves most about football is being able to run deep.

"Sometimes, I just take off and run down the field," he said. "It's just that feeling I get when I'm running."

But just as he gave up running for football, he someday will give up football for his first love: music.

In "Studio 89," which is his basement, he is working on what he hopes will be his first LP, a collection of his own contemporary jazz songs.

Muhammad plays "close to 12" instruments, and plays every single one that will appear on his record. His "group" is called "Saleem," which is Muhammad's middle name, taken when he became a Muslim in college.

"That's the name of the group, which is me," he said. "I am the group."

In Muhammad's junior year of high school in Jacksonville, the new drum major, a young man who admittedly "wasn't very athletic" as he grew up, quit the band to join the football team in midseason.

Muhammad says it's "the smartest move" he ever made.

For the time being. "I can't play football forever, so that's when I'll return to my music," he said. "But football is what I want to do now."