It’s early afternoon in Orlando, the hottest time of day on a characteristically sultry Wednesday in Florida, a state famous for its perennially warm, wet, shirt-soaking conditions, which just so happen to be pretty much perfect for playing tennis.

Or so I’m told.

I’m standing—sweating, squinting, panting—at the opposite end of a court from 21-year-old Ulises Blanch, one of the many elite athletes who train here at the United States Tennis Association’s National Campus. I’m here to learn about the upper limits of the serve, the most nuanced stroke in tennis and one of Blanch’s specialties. I tell him I’m ready. He toes the baseline, lobs the ball into the air, and sends it bolting past me. “One hundred thirty-one,” says the speed-tracking system. From across the court I see Blanch grin. Sadistically, I think. It’s his seventh serve, and his seventh ace.

Blanch possesses a tremendous serve, yet it remains far from the most powerful. It’s been clocked at 138 miles per hour, which 30 years ago would have put him in the running for the biggest hitter in all of tennis. But serve speeds at the professional level have been climbing for decades. The 1990s saw the first official serves in the 140s. By the early 2000s, they were in the 150s. The fastest serve ever recorded came in 2012, when Australian Sam Groth was measured walloping a ball at 163.7 mph. But the Association of Tennis Professionals doesn’t recognize Groth’s serve, because he delivered it at a challenger event, where, according to an ATP spokesperson, serve-speed guns don’t adhere to the same standards as the ones used in tournament play. The fastest serve recognized by the ATP was delivered at 2016’s Davis Cup by American John Isner, at a speed of 157 mph.

“There are three big factors in optimizing for speed,” says physiologist Mark Kovacs, an expert in serve mechanics. “Technique, technology, and height.” The sport’s latest generation of athletes, he says, have pushed the limits of all three.

A former tennis pro himself, Kovacs works with some of the best players on earth to help them wring as much power as possible from their serves. The technical elements of a stellar stroke, he says, are well understood. You need strength, obviously, but flexibility is equally important—particularly in the upper body.

During a serve, the majority of a player’s power originates in their legs, but conveying that power through the body and into the racquet requires stockpiling additional energy in the hips, lumbar, and shoulders, by rotating all three elements in sequence as the ball rises into the air. Tennis types call that rotation coiling. A big serve requires a limber, practiced player—someone strong and loose enough to twist their torso taught like a rubber band and uncoil themselves a fraction of a second later, with timing so precise that it not only translates the energy from their legs but augments it.

Hard hitters like Blanch excel at storing and releasing energy throughout their bodies in this way. But they’re also working with more power, in general. While a typical amateur might produce between 700 and 900 newtons of ground force with their legs, Kovacs says the most propulsive pros can generate upwards of 1,500.

There was a time when tapping into that kind of power on the court was risky. With older, wooden racquets, which dominated the game of tennis for much of the 20th century, serving too hard significantly increased your odds of overshooting, sending the ball out of bounds.

But in the mid-1970s, manufacturers began blending carbon fiber and resin to produce racquets with bigger heads. The surface area of your typical racquet increased from 70 square inches to well over 100. That expanded the racquet’s sweet spot, which made the game easier for amateurs. But the pros didn’t need a bigger sweet spot. For them, larger, modern racquets have had a different effect: The ability to put more topspin on the ball. Thanks to a phenomenon known as the Magnus effect, a ball with more topspin dives toward the court at a sharper angle once it has cleared the net. Putting more spin on their serves allowed players to lower their risk of launching the ball beyond the bounds of the service box when they hit harder. The result: more powerful play and faster serves—especially for bigger, stronger players.