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For much of major league baseball’s history, even the hardest-throwing pitchers rarely threw faster than 100 m.p.h. Those who did were accorded a certain fearful respect, like the villain entering an Old West saloon. But over the past decade, more pitchers have been coming closer to triple digits than ever before. Among pitchers who threw at least 40 innings in 2005, 11 averaged 95 m.p.h. or more, according to By 2015, the 95-m.p.h. club nearly quintupled: 54 pitchers averaged at least that speed. And last season, two dozen pitchers hit the 100-m.p.h. mark at least once–more than double the number who reached triple digits back in 2008. “A hundred miles per hour is no longer a mythical number,” says Jeff Passan, author of the new book The Arm: Inside the Billion-Dollar Mystery of the Most Valuable Commodity in Sports.

Why is baseball now a blur? One reason is specialized bullpens. Relief pitchers now often face only a batter or two, which helps keep their arms fresh. This has coincided with a revolution in training. Innovations like 3-D analysis help young pitchers improve mechanics, while more-scientific approaches to conditioning and nutrition have made them stronger. “The propeller-heads running sports now know if you do this and this with your movement, and this and this with your strength training, you can get to 100 m.p.h.,” says pitching guru Tom House, whose pupils have included Hall of Fame fireballers Randy Johnson and Nolan Ryan.

These days, the race to 100 starts young. Radar guns blanket youth baseball showcase events, where clocking a fast pitch is a surefire way to get noticed.

Heat, however, can have its consequences. Strikeouts per game reached an all-time high in ’15, while home runs spiked 17% year over year. Meanwhile, injury data shows that harder throwers are more likely to get hurt. Baseball has seen a spike in “Tommy John” elbow-reconstruction surgeries. “I’m not sure all this high velocity is sustainable,” says Hall of Fame hurler John Smoltz, a Fox Sports analyst. “Too many guys are going all out, all the time.”

Either advances in training and medicine will keep more big arms healthy–or more kids will flame out. In the meantime, prepare for more of the high cheese.

This appears in the April 18, 2016 issue of TIME.

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