If Los Angeles hadn't been struggling in early March, if Coach Pat Riley hadn't felt the urgency to shake up his players, if he didn't have a little urge to gamble, the Lakers might not have discovered the extra ingredient that has made them so successful in the playoffs.

The use of a trapping defense is one of the main reasons the Lakers can win the National Basketball Association championship here Sunday. They hold a 3-1 lead over Philadelphia in this best-of-seven series.

"They play the zone trap better than any college team," Julius Erving said today after the 76ers worked out at St. Joseph's College in preparation for Game 5 at the sold-out Spectrum (WDVM-TV-9 at 2). "In college, the way you bust a zone is to pass and wait for the good shot because you can take all the time you want. But we've only got 24 seconds, so we have to attack with definition."

In the trap, either Magic Johnson, Jamaal Wilkes, Norm Nixon or reserve Michael Cooper helps double-team the player with the ball. The trap has been effective because the 76ers have only one dependable outside shooter, Andrew Toney.

Riley says he might have used the trap a little in this series, but admits he couldn't have depended on it if he hadn't decided to emphasize the defense back in March, when times were tough.

During a span from Feb. 10 to March 12, the Lakers won only eight of 15 games. Riley felt he had to do something to motivate his players, to rekindle their concentration and stir their competitive fire.

"I thought we were getting soft," he said today before putting his team through what he hopes will be its last practice of the season. "I wanted to do something that would force them to think and make them work hard.

"In this type of defense, peer pressure won't allow you to rest. You can't just play standard man-to-man defense. You have to constantly be helping out. And if you don't, the other players will know who broke down."

Riley didn't foresee the enormous advantage the defense would give Los Angeles in this series. He says it was started strictly as a diversion, but when it became so effective he started stressing it more.

"It's a gambling type defense," he explained. "You can get burned, but we have some great athletes and that helps. Our guards have more size and quickness than most teams. It's a very active defense."

When Philadelphia guards Maurice Cheeks or Toney are trapped, or double-teamed by 6-foot-9 Johnson or Cooper and Wilkes, both 6-6, it often is difficult for them to see over the added defender and find the open man.

"It takes the other team out of what they want to do," said Johnson, who has done an excellent job of guarding Erving in the last two games. "We try to force mistakes, because when we get a turnover that's when our fast break gets going."

Johnson was the most valuable player of the championship series two years ago, when he scored 42 points in the deciding game as the Lakers beat the 76ers in six games. He certainly is in contention for that award again. The versatile three-year veteran leads the Lakers in rebounding (10.5 a game), and is averaging 18.5 points and eight assists.

"I don't think we could have asked anything more of him," Riley said. "We've played him at different positions, put him on different people. He's gotten us points, rebounds, assists, whatever. And he's been very effective in our defense."

When Riley started emphasizing the trapping defense, Johnson liked it because it is designed to create turnovers and get him out on the break. However, he adds with a smile, he didn't think it would be so valuable in the playoffs.

"The first time it helped us was against Phoenix," Johnson said. "The Suns like to set up, but they couldn't do it as much when we trapped the ball. Also, it got us running, and once we get a lead we're hard to beat." Wilkes also was skeptical when so much time in practice was devoted to the trap. "I didn't know if it would work on a regular basis," the all-star forward said. "But it really forced us to think more about defense and that probably was the motive. You can't loaf, that's for sure."

Kareem Abdul-Jabbar, a veteran of 12 seasons, shrugged off any effect the new defense might have had in the Lakers' late surge, when they won 15 of their last 19 games.

"It was a new wrinkle," he said. "Maybe it picked up our intensity some, but I think the team just came together at the end of the season. We've matured as a group and we're playing as well as we have all season. We'd like to end this thing right now."