TAMPA, Fla. — With spring training camps opening in Florida and Arizona, after a baseball season in which the average major league game lasted a record 3 hours 2 minutes, Major League Baseball and the players’ union took their first formal steps on Friday to quicken the pace of play.

Major League Baseball and the union announced new initiatives, including countdown clocks for breaks between innings and modifications to the instant-replay process. Baseball also said that umpires would enforce the rule requiring all hitters to keep at least one foot in the batter’s box.

“Once I’m there, I want to hit, I want to do my job, but other players normally take their time, come out and do certain things in order for them to feel comfortable,” the Yankees’ Carlos Beltran said. “It probably will hurt some other hitters.”

Hitters can leave the box under certain circumstances, including a wild pitch, a play on the bases, even a swing. But if the umpire believes the batter is intentionally stepping out and delaying play, he can call a strike without the delivery of a pitch — or, at his discretion, offer a simple warning.

Commissioner Rob Manfred said in a statement that enforcing the batter’s box rule would “help speed up a basic action of the game.” He also said the most important starting point for improving pace of play was moving efficiently in and out of commercial breaks. Two clocks will remind fans and players of that effort.

Major League Baseball installed a pitch clock at a ballpark in the Arizona Fall League last October, and officials were pleased with the results. Sandy Alderson, the Mets’ general manager, said the clock there did not adversely affect the way the game was played.

The new clocks will not count down the seconds between pitches; those clocks are being installed at Class AA and AAA parks this season, but not in the majors. In major league parks, there will now be two timers that will start a countdown immediately after the third out of an inning. One will be on or near the outfield scoreboard, and another on the facade behind the plate, near the press box.

“It’s a big undertaking, too, getting these timers installed at various places at Double-A and Triple-A, and also at the major league level,” Alderson said. “But M.L.B. has been pretty aggressive on this point, and I’m pleased the players’ association has agreed to some of these changes.”

The clocks in the majors will count down from 2 minutes 25 seconds for games on local television, and from 2 minutes 45 seconds for games on national TV. Pitchers may throw as many warm-ups as they want, but must stop throwing with 30 seconds to go. Hitters’ walk-up music will be played between the 40- and 25-second marks only.

If a pitcher does not deliver a pitch before the clock hits zero, the umpire can issue a warning and a fine. Baseball said there would be no fines issued in spring training or this April as players adjusted to the rules.

“Pace of game is something baseball has been addressing for years, and I know it’s very important to the commissioner and the owners, and we have to respect that and try to do the best we can, and hopefully it is effective,” Yankees Manager Joe Girardi said. “I think people just don’t want as much downtime in between innings and in between changes, and that’s what they’re really striving for.”

Managers will also be affected in their method of challenging a call. Last season, the first with widespread instant replay, managers routinely left the field to feign an argument with the umpire while keeping an eye on the dugout. Behind the scenes, a team video coordinator would study the replay and tell a coach whether the manager should issue a challenge. The manager would not leave the field until getting his signal from the coach.

Those tactics were not part of the rules and served as a tedious sidelight to an otherwise effective replay rollout. Managers can still rush onto the field to challenge a potential inning-ending out. But in other instances, they are now instructed to stay in the dugout and signal to the umpire from the top step — verbally or with a hand signal — that they are considering a challenge.

“That’d be great,” Beltran said. “Managers don’t need to go out. Once you know you should challenge, you should raise your hand, umpires should go take a look and that’s it. We want the game to continue; we want the game to move.”

So does Manfred, who has prioritized the idea of making baseball more appealing to a generation craving immediacy.