"Thwump!!"

The force of leather hitting flesh made a loud, dull thud. And even Floyd Patterson, who has heard that sound a few times before, had to flinch at its realism.

"Thwump!!"

Patterson, once heavyweight champion of the world, watched and winced as Sylvester Stallone, playing challenger Rocky Balboa, and Carl Weathers, as the Muhammad Ali-like champion, Apollo Creed, swapped haymakers in 12 minutes of the the most brutal fight footage you'd ever want to see.

It came in the climax of Rocky II, the sequel film to the 1976 Academy Award winner, which opened this weekend and may just out-Rocky the original.

"Thwump!!"

Both men finish the film with their faces puffy; swollen testimony to the fierce bout they have just waged. Much of it is, of-course, makeup magic, but Stallone and Weathers did connect with a number of the blows.

"I wanted people to understand what it's like to be hit," said Stallone, who wrote and directed Rocky II. "You know, you hear people call some fighters bums. Well, any man with the nerve to walk into a ring, to put his life on the line, that takes a lot of guts. There's no such thing as a boxer who's a bum. I have great respect for them."

It is from that respect that Stallone's character, Rocky Balboa, is born. Rocky is a rough-around-the-edges Philadelphia club fighter who is the perpetual underdog. And as fine a job as Stallone does with that role, Weathers almost steals the show as Creed, the flamboyant champion who beat Balboa in a bloodbath in the first Rocky.

Weathers is a former pro football linebacker who spent 1 1/2 years with the Oakland Raiders and was on the field when Ben Davidson got into his famous war with the Kansas City Chiefs in 1970. "When it started, I just walked over to the bench and sat down," confessed Weathers.

The character of Apollo Creed is clearly borrowed from Ali and Weathers studied films and pictures of the champion. "I had never met Ali," Weathers said. "Then, after the first Rocky, I was in a hotel in Beverly Hills. There was a whole lot of commotion outside, a big crowd, and all. I looked out and it was Ali. It was the day he was getting married to Veronica Porche.

"I went outside and started shouting and pointing at him, yelling, the way he would, screaming, ‘I want your autograph!... I want your autograph!'"

Ali wheeled and saw Weathers. He pointed right back at the actor and shouted: "It's Apollo Creed!"

They embraced and Ali told Weathers that he thought the characterization of him was outstanding. "That’s the compliment you want," the actor said. "When Muhammed or Floyd Patterson say you did a good job, that’s very satisfying. That’s first-hand knowledge."

Weathers succeeds in creating some sympathy for Creed, who was the heavy in the original Rocky. "It was tough to pull it off, to give both sides of the coin. Give the credit to good writing and good direction."

Both of those jobs were done by Stallone, who approached the project somewhat warily.

"You're talking about making a film that will be compared to one which won an Academy Award," he said. "I didn’t want a Planet of the Apes series. Sequels are usually bad. But a miracle happened for us. All the people from the first Rocky came back."

Missing, however, was the original director, John Avildsen. "He didn't like the script," said Stallone. "He suggested I do my own directing, so I did."

It is no easy task, but Stallone pulls it off. There is one marvelous sequence of the children of Philadelphia racing through the streets with Rocky while the fighter does his roadwork.

"We advertised for any child who wanted to run with Rocky to come down," said Stallone. "Well, about four kids and 99,000 adults showed up. You know, guys from the Philadelphia Striders (track club). They were beating me. So we said, 'OK, you beat Rocky.’ Now will you please bring your kids down?' When the 13-year-olds showed up, they were beating me, too. Finally we got some 9-year-olds and Rocky beat them."

Weathers approached Stallone as a director with some doubts.

"He kept saying, ‘Trust me, trust me,’ and all the time he’s hitting you."

Stallone said ring realism occasionally led to trouble in the filming of Rocky II. One of his sparring partners, for example, was former lightweight champion Roberto Duran.

"A few times, it got out of control," he said. "You’re sparring and you say to yourself, ‘Hey, maybe I can go with this guy.’ Then you trade a few shots, and you realize you’re an actor again."

Will there be a Rocky III?

"I think so," said Stallone, who already has started forming a story line. Weathers will be back for that one, and when Patterson heard that, he asked a favor from the actor.

"Hey, Carl," he said. "In the next one, a little more defense, please."