The start of what could become the most volatile negotiations in the history of professional athletics will take place today when representatives of the National Football League and the NFL Players Association meet in Hollywood, Fla.

The two sides will begin discussing a new collective bargaining agreement to replace the current contract, which expires July 15. A major issue will be the NFLPA's unprecedented demand to begin sharing in the league's annual gross revenues.

Although the principal NFLPA requests have been known for almost a year, this will be the group's first formal presentation to the league. But little is expected to happen during this scheduled four-day session.

Months of rhetoric on both sides has led most observers to one conclusion: the players are headed for a strike, probably around the opening game of the 1982 season, unless some compromise is reached. That compromise now seems unlikely.

"We've got to convince them that the players are deadly serious about percentage of the gross," Garvey said. "I'm sure they are going to try to convince us that they . . . won't negotiate that point.

"But at one point, they are going to have to talk to us about it. I've seen enough times when they said they would never negotiate something with us and then they have. The worst thing you could say in labor negotiations is never. This is what the players want. We are unified about that."

The league, speaking through chief negotiator Jack Donlan, already has said it never will negotiate that demand, even if the players strike because of it.

The NFL's approach will be governed by how it judges player unity. In 1974, when the players struck over contract negotiations, the walkout ended quickly and it took three more years for the present contract to be worked out. The irony now is that it may take another strike for the players to prove to the owners that they no longer have the same internal problems, that they no longer are fragmented, that making Garvey the negotiating target, as has happened so far, will backfire.

To reinforce their unity, the players and Garvey have been preparing for this day almost from the moment the 1977 agreement was signed. They have affliated with the AFL-CIO, they have conducted a series of regional and team meetings, they have prepared videotape instructional films and they have compiled extensive data reflecting what the players want. Garvey says data show that well over 90 percent of the players want percentage of the gross in the new contract.

"But the league doesn't seem to be in any hurry," he said. "It's just the same attitude baseball took. What's the rush? Are they trying to force a strike? Will the public stand for that?

"What will make them speed up? Well, I think the television networks eventually will apply some pressure. They are negotiating now. In fact, I've heard they've already reached an agreement that will bring each team $12.6 million a year, but I can't confirm that. But if there is no certainty there will be a season next year, the networks are going to get worried about it after awhile."

It also may take awhile for the league to absorb all the facets of the NFLPA'S main demand. The NFLPA has been talking about wanting 55 percent of the league's gross revenues per year, but the exact percentage won't be worked out until its convention, starting March 21.

If it obtained a percentage of the gross revenues, the NFLPA would pay the players according to years in the league, regardless of position. Using 1980 average salaries and gross revenue, first-year player salaries would increase from $51,087 to $75,000. Third-year players would increase from $67,868 to $105,000 and so forth. There also would be many incentives. Players making the Pro Bowl would get a bonus. There would be performance bonuses, such as for being a regular-season starter. And playoff teams would get a sizeable chunk of the revenue pool.

From the gross pool also would come money to pay increased health and accident benefits, player pensions and other programs, some new under the NFLPA proposal.

Garvey says the percent-of-the-gross demand is necessary because "nothing else will work in the NFL. There is no incentive to win. Being a free agent in the league is a joke. This system will benefit both the players and the league. It will do away with individual bargaining and renegotiating, it will do away with unsavory agents. It will bring stability."

The NFLPA's attempt at bringing free agency to the league, in the 1977 agreement, failed. Only one player changed teams under the present system, which Garvey and his group helped devise. While other league salary averages grew dramatically, football's has lagged despite the popularity of the sport. (In 1980, the average NBA salary was $186,000, baseball $143,000, football $78,000.)

Donlan, however, says that percent of the gross means only one thing to the owners: control. "They (NFLPA) want to help us run the league and that's not going to happen," he said. "We agree there could be improvements, we agree they could be paid more money and we are willing to talk about it. But not percentage of the gross."