Some describe it as a whip cracking through the air. Others call it a powerful spring unwinding. Some liken it to a catcher firing to second base.
But the best summary of Andy Roddick's arm as he uncorks another titanic serve comes from the numbers: 149, or maybe even 150. As in miles per hour.
Roddick, who lost in the semifinals of Wimbledon Friday, served up a 149-mph bullet June 14 at the Queen's Club tournament in London, tying Greg Rusedski for the fastest serve since radar guns began clocking them in 1991.
Conventional tennis wisdom says it's only a matter of time until he hits 150 mph and Roddick's coach, Brad Gilbert, told ESPN.com last week that 160 is definitely in reach.
The 20-year-old American is in the vanguard of a generation of big hitters who have been pushing radar dials into new territory over the last decade. Scientists and trainers say these rockets are more than the product of modern equipment and basic weight training. Today's power servers take advantage of the latest research in biomechanics to push the human body and the game of tennis to the limit.
"There is a perception that [the serve] is getting faster, and it is probably true," said Duane Knudson, a professor of biomechanics at Chico State University in California.
Between 1991 and 1996, the fastest serves recorded on the men's tour ranged between 131 and 137 mph. Since then, the numbers have ranged from 140 to 149 mph (measured as the ball comes off the racket).
Although the highest numbers are posted by a few top gunners, Bobby Bernstein, who has coached the nation's top juniors for USA Tennis High Performance, said 20 to 25 players on the men's tour can land a first serve in the 130 mph range. Ten years ago, he said, there were "maybe a handful."
Bernstein, who coached Roddick in 1998 and 2000, said he has noticed a similar increase on the women's side, where serves reach the low 120s.
A 149-mph serve like Roddick's takes just under half a second to reach the opponent's baseline, according to Howard Brody, a retired physics professor and occasional tennis coach.
That split second was enough time for Andre Agassi, the world's best returner, to send back Roddick's offering. But Brody said returning serves of that speed is probably a matter of good guesswork.
In fact, Chico State's Knudson believes the service game is pushing the current rules of tennis to their limit. If speed increases another 10 percent, he says, "you would have to change the rules because people would not be able to return the serve." Some critics are already calling for change, arguing that 130-mph bullets have killed the rally. And the International Tennis Federation has approved a larger ball with greater air resistance that could slow the game. But the pros - and almost everyone else - have ignored it.
Players such as Roddick, Rusedski and Taylor Dent, all of whom have broken 140 mph, are the evolutionary winners in a process that began when Baltimore's Howard Head enlarged the size of the tennis racket in 1976. That gave every player a bigger sweet spot and made even marginal hits faster.
Since then, larger and longer rackets, with lighter and stiffer frames, have allowed players to swing much faster, resulting in even higher velocity. But scientists say those breakthroughs are at least a decade old - they don't explain the emergence of Roddicks and company.
So what does? In terms of physics and geometry, height is a major advantage, according to Brody. Roddick and most other heavy hitters are at least 6 feet tall. They can take advantage of a straight line that runs from the impact point of their serve to the opponent's service line, clearing the net along the way. Their shots don't need gravity to land inside the service box.
On the other hand, Brody said, there's a limit to the speed of a shorter player's serve since gravity must pull the ball down into the service box before the ball's velocity carries it out.
But hitting 140 mph takes more than altitude. "It's their height that gives them the window," Brody said of the hard hitters, "but it is their biomechanics, it is their athletic ability, it is their timing" that makes a big serve happen.
If there's any limit to the speed of the serve, it's entirely human. "Limits are put on by the tolerance of the body tissue," said Bruce Elliott, an Australian biomechanist who has worked extensively with tennis players. "Are we getting near those? We probably in some respects are."
Elliott and other biomechanists have conducted research showing that little muscles can provide big returns. With the aid of high-speed cameras and three-dimensional imaging technology, Elliott has identified the internal forward rotation of the upper arm as a key "power generator." It's a movement that tennis players have in common with baseball pitchers.