Nearly 30 percent of pitchers in Major League Baseball have undergone Tommy John surgery, the revolutionary elbow-ligament replacement surgery named after the pitcher who first underwent it in 1974. No longer is the malady, which experts and doctors are calling an epidemic, limited to professional players. In The Arm: Inside the Billion-Dollar Mystery of the Most Valuable Commodity in Sports, author Jeff Passan finds a frightening trend in young arms getting hurt and a youth baseball culture too ignorant to stop it.

Doctors consider Tommy John surgery one of the most successful medical procedures ever because it solved a problem. When an elbow ligament tore, it could be fixed. Baseball rejoiced. “We thought elbows were solved,” former Red Sox general manager Ben Cherington said. “So we stopped thinking about them.”

Because an answer for elbow issues existed, the sport never bothered to concern itself with the root cause of such injuries. Maybe it was mechanics, the way a player throws the ball and its effect on his body. Perhaps it was usage, the volume of pitches or innings in a single game, over a whole season, or even longer. Certainly a player’s genetic makeup could factor in, too, or how hard he threw, or what pitches he preferred, or his diet, or any other sort of measurable factor.

Tommy John surgery, it turned out, was a paradox, the procedure that worked too well. It lulled baseball into a false sense of security, and by the time the sport realized what had happened, an epidemic was on its hands. Elbows are breaking more than ever and younger than ever. And while the rash of Tommy John surgeries that spread across Major League Baseball over the last five years took out some of the game’s finest pitchers, children ages 15 to 19 make up a disproportionately high number of patients. Baseball is thus left scrambling to figure out how to keep its million-­dollar arms healthy while fixing a feeder system that keeps sending damaged goods to major league teams.

At a youth-baseball complex southeast of Phoenix, a ten­-year-­old boy named Harley Harrington stood on top of a mound and twirled pitch after gorgeous pitch. Harley’s motion was a study in biomechanical beauty, his legs driving efficiently, his hips swiveling at just the right time, and his right arm unfurling so smoothly it looked machine­-taught. His peers chucked the ball; Harley delivered it.

He came here in March 2015 from San Diego with a traveling baseball team called the Show, which recruited some of the best ten-­and-­under kids in Southern California to compete in high-­level tournaments like this one, the Spring Championship Super NIT in Gilbert, Arizona. Hundreds of other teams in all age groups, some as young as seven years old, came from around the country to feed the excesses of American youth baseball personified by the Big League Dreams complex. Built near a farm, it reeked of cow dung. Local politicians still kick themselves for spending more than $40 million to develop the campus for the private company that runs ten more such facilities across the West Coast.

Four fields, each built in the scaled-­down image of a famous major league stadium, surrounded a central hub of video games, flat-­screen TVs, bad food, and, most important, copious beer. The taps started flowing around eight a.m., when some fathers lubed themselves to forget they’d been conned into traveling hundreds of miles for games that just as easily could’ve taken place ten minutes from their houses. The youth baseball-industrial complex can hypnotize even the most mindful.

Nicola and Martin Harrington never expected to find themselves in a facility like this. Nicola once was a pop star in England whose band, the Simon Cowell-backed Girl Thing, fizzled amid great hype. Much of the drama involved Nicola’s secret relationship with Martin, a music producer. They married, had Harley, left England, and ended up in Los Angeles, where a friend of Martin’s told him that now that his boy was American, he needed a baseball glove. Harley fell in love with the game and showed enough aptitude that he craved better competition.

While Harley started as an outfielder, his coaches quickly recognized the fluidity with which he threw a ball. Pitchers spend a lifetime trying to look as natural pitching as Harley did the first time he stepped on a mound.

“Having been around some really good players in our program, sometimes we single out kids who remind us of others,” said Hector Lorenzana, one of the Show’s longtime coaches. “We see flashes in things kids do at certain ages. And it reminds you of other players who have come through. Harley is one of those.”

At the Spring Championship Super NIT, whose champion qualifies for an even bigger tournament later in the year at Disney World, the Show ran roughshod through its bracket to reach the semifinals, where it unleashed Harley. He mixed fastballs and off­ speed offerings, all from the same release point, each pitch faster and crisper than his peers’. Harley exited in the fourth inning after 52 pitches, well short of the tournament limit of eight innings with no maximum pitch count. Martin always kept track of how many Harley had thrown, and when the Show squeaked out a victory to get into the finals, he approached Lorenzana about Harley’s availability for the next game. Lorenzana was hesitant.

“Going in the same day back-­to-­back,” Lorenzana said, “is a huge no-­no for us.”

And yet Martin Harrington, conscientious enough to download an app to track his son’s usage, a voracious enough reader to realize that the rash of Tommy John surgeries points back to excessive and unnecessary throwing by children, wanted his son to pitch in the final game, if need be.

Another coach lobbied Lorenzana, too, pointing to Harley’s parents. “Look at the mom! Look at the dad!” he said. Both were pictures of fitness. Also working in Harley’s favor was that unlike almost every top travel­-ball player, he actually took time away from baseball, spending his summers in England or playing club soccer. The single­-sport­-specialization malady that affected kids across the U.S. landscape did not apply.

Still, nobody knew. Not Lorenzana, not Nicola or Martin Harrington, not the doctors urging coaches and parents to pump the brakes on excessive use. Nobody could say whether putting Harley in for a second time would cause damage years down the road. Every kid and every arm is different.

At 4:15 p.m., in the fifth inning of a blowout game the Show led, about two and a half hours after he had last pitched, Harley Harrington went back out to win a tournament for a group of ten-­year­-olds.

“If it was anyone other than Harley, we’d have shied away from it,” Lorenzana said. “There are some horses you’re going to ride a little longer. There’s no science. There’s no process. You just don’t know.”

Increasingly, researchers are looking beyond the major leagues and down to kids like Harley Harrington and how they’re being handled. Grave concern exists among those studying the arm that because of tournaments like the Super NIT, which lack pitch counts, the current generation of injured arms will look positively healthy compared with the kids’ coming up.

The American Sports Medicine Institute (ASMI), the baseball industry’s foremost think tank, followed nearly 500 youth-­league pitchers for a decade starting in 1999 and found that kids who pitched more than 100 innings in a calendar year were three and a half times likelier to get injured than those who didn’t. In 1997, Dr. James Andrews, the famous orthopedic surgeon who had founded ASMI in Birmingham, Alabama, was performing Tommy John surgery on one or two high school kids per year. Today, he estimates he does 80 or 90 a year. “Hell, I’ve got four to do tomorrow,” Andrews said during an April 2015 conversation. He fears that even worse news is coming at the major league level. “If they don’t get involved in it from a prevention standpoint at the youth level,” he said, “they’re not going to have anybody to draft out of high school or college who hasn’t had their elbow operated on.”

The future generation of baseball pitchers lives in a system that takes undeveloped and underdeveloped arms and pressures them to show off for the radar guns they’re taught will determine their future. The easiest way to build velocity is through year-round throwing—and year-­round throwing, according to the ASMI study, is the single highest predictor of future injuries among kids. Risk factors are highest for kids like Harley, whose arms are especially fragile at ten years old and, in many cases, remain so through the end of high school and beyond. Some surgeons have performed Tommy John on kids as young as 13 years old, even as doctors at the top of the field warn against cutting still-­growing arms. Children who regularly pitched with arm fatigue are 36 times likelier to undergo elbow or shoulder surgery, another study by Andrews and his peers at ASMI found.

UCL reconstruction is far from foolproof, too. The procedure involves cutting through skin and muscle, drilling into bone, and tying the elbow together. It is major surgery that calls for a brutal, monotonous rehabilitation. And while the return rate is around 80 percent, a study from Jon Roegele at the Hardball Times looked at the return of every pitcher who underwent Tommy John surgery from 2000 to 2009 and found the median threw just 60 games and 100 innings for the rest of his career.

There is nothing glamorous about Tommy John surgery. The urban legend of doctors performing it pre­emptively and prophylactically is unfounded. Forget another myth, too: the problem stems from kids throwing curveballs too young. Another ASMI study showed that curveballs cause less strain on the arm than the simple, humble fastball, whose greater velocity taxes pitchers more. In 2003, the average fastball in the major leagues didn’t crack 90 miles per hour. Today, it’s over 92, jumping annually for eight consecutive years and placing not just a physical burden on every kid who dreams of being a big leaguer but also a mental one: throw hard or your chances are grim.

So they travel like Harley Harrington, using pitch-­all-­you-­want tournaments to ready themselves for the grind of their teenage years, when scouts will converge on showcase events to see kids who have been reared to do everything bigger, faster, harder.

“Travel baseball is completely different than it was 20 years ago,” said Paul DePodesta, now the chief strategy officer for the NFL’s Cleveland Browns after spending two decades in baseball front offices. “With all the showcases and these guys pitching, it’s not just when they’re 17. It’s when they’re 14 and 15.” DePodesta has four kids, three boys. His second son, Evan, played on an all­-star team that was invited to a regional tournament in 2014. He got to stay in a hotel and wanted to keep doing that with a travel team. Sure, his dad said, except he might have to give up football and soccer and basketball.

“I don’t think I’m ready to choose,” Evan said.

“Well, you shouldn’t,” DePodesta said, “because you’re six.”

The Arm. Copyright © 2016 by Jeff Passan. Reprinted with permission of Harper, an imprint of HarperCollins Publishers.