Blake Billinger began playing baseball year-round at age 10, when he joined a club team and competed in tournaments nearly every weekend.
Two years later, his Silicon Valley Yankees climbed in the national rankings and started traveling -- from Los Angeles and Las Vegas to Florida and Kansas City.
Billinger pitched five to seven innings most weekends, but he always figured he rested enough. Even in the summer after his freshman year at St. Francis High in Mountain View, Calif., when tendinitis in his right shoulder sidelined him, Billinger was not especially worried.
Then his elbow made him worry -- in a big way.
On Oct. 20, 2012, at a showcase event for college prospects in Arizona, he threw a pitch and instantly felt "a huge pop." His right elbow screamed in pain. Billinger dejectedly walked off the field, fearing for his baseball future.
Barely a month later, at age 16, he had reconstructive surgery on his ulnar collateral ligament, better known in baseball circles as Tommy John surgery. The procedure has long been common for major-league pitchers with years of wear and tear on their arms, but Billinger's case offers a stark reminder: Teenagers are vulnerable, too.
It's just one of the many side effects of specialization in youth sports. As more and more kids play the same sport year-round from an early age, they are increasingly vulnerable to injury.
This trend toward focusing on one sport can sharpen skills and even set young athletes on the path to scholarships and college success. But it also means more repetition, more strain and more injuries.
Like many of the issues surrounding specialization, experts and athletes alike question whether the trend is in the kids' best interests.
The numbers are stark. There has been a fivefold increase since 2000 in the number of serious elbow and shoulder injuries among youth baseball and softball players, according to the American Orthopaedic Society for Sports Medicine.
The organization helped launch StopSportsInjuries.org, a website devoted to educating parents and young athletes about sports injuries.
Among the sobering statistics: Overuse injuries are responsible for nearly half of all sports injuries to middle school and high school students. A report this year by the sports medicine department at Loyola University of Chicago found that "kids are twice as likely to get hurt if they play just one sport as those who play multiple sports."
None of this surprises Robin Bousquet, a senior physical therapist at the Sports Medicine Center for Young Athletes at Children's Hospital in Oakland. The injury spike is reflected in the evolution of the buildings where she helps sore, overtaxed kids.
In 2003, when Bousquet started in the orthopedic department at Children's, the clinic where she worked was the size of four trailers. Now the hospital has two huge clinics exclusively dedicated to treating young athletes.
Dr. Kirk Jensen, an orthopedist in Oakland, recalled the time one father brought in his son, then 12 or 13. The kid was right-handed, but the dad had made him pitch left-handed, knowing lefties are considered more valuable. He couldn't lift his arm because of shoulder pain, and his velocity had been dropping, according to the all-important radar gun at home. But the dad wanted Jensen to clear the kid to pitch in an upcoming all-star game.
Jensen, naturally, told him he needed rest. Dad, naturally, got mad.
Jensen is a shoulder specialist, so he sees plenty of young baseball players who throw too much. He also runs a youth water polo program, so he sees swimmers and water polo players with shoulder injuries, too -- plus many young soccer players with knee injuries.
His water polo club encourages kids to play other sports. Not long ago, during a 12-and-under practice, one water polo player approached Jensen to tell him he had to leave early for a basketball game.
Rather than scold the kid or demand his undivided sporting energy, Jensen said, "Have a good game."
This stems from his experience with young athletes who not only sustain injuries but also endure what Jensen calls emotional burnout. Such kids dedicate themselves to one sport starting at age 9 or 10, reach their junior or senior year of high school -- and then don't want to play anymore.
"Personally, up until age 14 or 15, I think kids should play as many sports as they can," Jensen said.
It's a tricky balancing act, given the increasingly fierce competition to make high school teams and the potential pursuit of college scholarships. Dr. Marc Safran, associate chief of sports medicine at Stanford Medical Center, knows the benefits of playing multiple sports, but also understands the need to eventually specialize.
There's no foolproof formula, though Safran warned of the danger of playing some sports, such as golf, tennis or baseball, year-round at young ages. Those are what he called "unilateral arm-dominant sports," making proper technique and proper rest all the more essential.
Or, as Bousquet said, "No one was born to throw a baseball overhand 100 times; doing that is not an inherent thing for humans. And now we're going to make 10- or 11-year-olds do it year-round?"
Billinger insisted his coaches were careful with him, but many coaches don't keep pitch counts and place more emphasis on winning.
Billinger, now 17 and a high school senior, also played basketball and flag football in middle school, but baseball was a "full-time commitment." He attracted the attention of recruiters from major-college programs across the nation -- at least until that day in Arizona when his elbow popped.
He still doesn't regret the long seasons and frequent travel when he was younger, mostly because he had fun. Still, Billinger acknowledged he wasn't educated enough about arm maintenance; he wishes he had done more preventive exercises and thrown three to five innings per weekend, instead of five to seven.
"I was totally naive to arm problems when I was younger," Billinger said. "I thought I was invincible."
That's exactly the kind of youthful bravado that Dr. James Andrews, the noted orthopedist, has tried to counter. One of Andrews' specific guidelines: Young pitchers shouldn't begin throwing a curveball until they begin shaving.
Among Andrews' other baseball recommendations are focusing on proper pitching mechanics, taking three to four months off each year, and avoiding the use of a radar gun at a young age because it leads to overthrowing.
Other sports also are prone to overuse injuries. Stopsportsinjuries.org includes guidelines for sports ranging from cycling and lacrosse to swimming and volleyball.
Bousquet, the physical therapist, finds particular frustration in the abundance of torn anterior cruciate ligaments in youth soccer. Those knee injuries might not trace directly to overuse, but Bousquet contends kids need more strength in their hips and core to support frequent pivoting.
Billinger's story might still have a happy ending: Billinger accepted a scholarship offer this past summer to play college baseball at Florida International. It doesn't hurt that he stands 6-foot-4, weighs 225 pounds and wields a mighty bat.
Still, he now knows he's not invincible.