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It's all in the swing specifically, it's in the velocity of the swing. According to physicists, a ballplayer needs to swing his 33- to 36-ounce ash bat about 70 to 80 miles per hour to defeat a 90 mph fastball. If hit squarely, this combination will send the ball about 420 feet at about 110 mph. That would clear all but a handful of Major League center field walls.
As Robert Adair says, "The whole trick is to swing the bat fast." Adair is a retired Yale physics professor and author of The Physics of Baseball. To achieve that and hit Major League pitching, he says, "you want to keep your swing compact and efficient."
The power needed to generate a quick swing comes from a very specific chain of movements. It involves the whole body, from the legs up, and "not just one part like the hips or the wrists," says Glenn Fleisig, a biomechanist at the American Sports Medicine Institute. "It's a well-timed sequence of motions."
A power swing starts with a stride forward with the front foot and a push against the ground with the back one. At the end of the stride, the batter rotates his pelvis first with his shoulders closed, then opening them up toward the pitcher. As the shoulders rotate, the arms delay. Then the arms come forward and the wrists are delayed. Then the hitter will cock the bat turn his wrists for the final follow-through.
"A quality batter will cock his bat later," Fleisig says. "You need to keep all these parts of your body moving one after the other, in that order. This is the secret."
The other part of this secret formula is reaction time. The faster the reaction time, the longer a batter can wait before swinging. That way, explains Adair, "you can get more information on the ball."
But don't wait too long. Subtracting four feet from the mound to the plate (60.5 feet) to account for the pitcher's stride, a 90 mph fastball takes 0.43 seconds to meet the batter from the pitcher's fingertips. Not much time to think and swing. A Randy Johnson 99 mph fastball will come up on you in 0.38 seconds. In order to react and get a good swing in that tiny interval, a hitter needs a light bat, a short stride, and a compact swing.
McGwire, who uses a light, 34.5-inch, 33-ounce bat, is able to take a shorter swing without all of the power-generating movements and make up for it with his bulk and raw physical strength. (ESPN, using its latest measuring gizmo, clocked McGwire's bat speed at 99 miles per hour faster than any others.) A smaller guy, on the other hand, needs to take a longer swing, with a heavier or longer bat.
"He needs to put a smaller force over longer time to get the same effect a bigger guy could do with a shorter swing," Adair says. "But then he is at a disadvantage. He has to start the swing sooner and gets less information on the ball. Ken Griffey, a slightly smaller guy than McGwire, has a beautiful swing, but it's longer."
Small or big, these elite athletes have more " 'twitch muscles' that allow batters to accelerate their bats more quickly," says James Ashton-Miller, professor of biomechanical engineering at the University of Michigan. "The other types of muscle fibers are endurance fibers, which react less quickly. So even if you have two people of identical sizes, the composition of their muscle fibers could make a big difference."
And exactly how much force is being placed on a home-run ball? "It can peak at 10,000 pounds of pressure," says Adair. Think of it as four midsize sedans sitting on top of a baseball. "But remember that's only for about one one-thousandth of a second." If a McGwire hit were filmed with a high-speed camera and played back in slow motion, the baseball would be shown compressed to half its size when in contact with the bat.
"That heats up the ball as well," Adair says. "Appreciably . . . right where you hit the ball, a couple of degrees Fahrenheit. And a hot ball has more elasticity. It goes farther."
Elasticity, or how high a ball can bounce, is what makes the ball go out of the park. The more elastic, the farther it will go. A baseball dropped from a height of six feet will only bounce back about three. A golf ball, which has higher elasticity, will come up a few feet higher than that.
Also, the higher the ambient temperature on game day, the better for the batter. The colder it is, the better it is for the pitcher. (Old baseball trick: Pitchers used to store their balls on dry ice in a cardboard box and take them out an hour or so before the game so their skins would warm up, but their cores would remain cold. Homers would turn into pop flies.)