Football running backs worry about their knees, baseball pitchers nurse their arms and golfers fret about their backs.

Today, Jack Nicklaus was forced to withdraw from the Masters before the second round because of a reoccurrence of the lower-back muscle spasms that have bothered him for the past four seasons.

Nicklaus, who began the day in 42nd place after an opening-round 73, felt stiffness, then pain, in his back when he hit practice balls. Thinking a walk might help loosen him up, Nicklaus went to the practice putting green, but, after a few putts, found he couldn't bend over to get the ball out of the cup.

Nicklaus, who said this was the fifth time since 1980 that he's had this problem, went to the locker room, where playing partner Gary Player gave him a Butazolidin pill. Nicklaus reported to the tee (the 10th) to begin his round, but, after three practice swings, said, "My back spasmed up . . . "

By the time he returned to the clubhouse, Nicklaus was looking pale and had some difficulty climbing a flight of stairs. However, after lying down for a few minutes, he regained his color, could move without pain and even joked with the two men attending him.

"It's exactly the same thing I had at the World Series (of golf) in 1980," Nicklaus told pool reporters as he lay on a couch in the Champions Room of the Augusta National clubhouse. That '80 World Series was the only other occasion on which Nicklaus' back problems have forced him to withdraw.

"Basically, I have felt a little discomfort in my back for about a week . . . tried to get rid of it at home on Monday," Nicklaus said. The man Nicklaus usually sees for his problem was "afraid to move anything," since this was Masters week. "He did work on it a little, and it did help."

Nicklaus, the only five-time Masters champion, says that he still plans to play in the Tournament of Champions at La Costa, Calif., the week after next, and he insists that doctors have told him that "there is nothing wrong with my back."

In addition to his two withdrawals, Nicklaus had the problem at the '81 World Series, at Pinehurst one year and last fall just before going to Paris for exhibition matches.