Alex Apfel planted her leg to take a free kick, and the pain followed. The growth plate on her tibia had been pulled off by the tendon, creating a break in her leg that would take her away from the field for five months.

It wasn’t a random injury, doctors would tell the then-13-year-old. It was the result of Osgood-Schlatter disease, an inflammation of the area just below the knee, where the tendon from the kneecap (patellar tendon) attaches to the shinbone (tibia) — and is caused by repetitive motion.

Now a 17-year-old senior at Fieldston High School, she initially pushed through the discomfort, describing it as “little pains” that wouldn’t go away. From the age of 5, soccer was her only sport. She would play it 10 months a year, kick after kick after kick. To the point of overuse.

“It was obvious,” her mother, Natalya, said in a phone interview.

Apfel, who has also suffered numerous other injuries, such as a pelvic fracture and slight labrum tear in her hip, is not an anomaly. Several local coaches and multiple doctors believe as more and more young athletes are specializing in a specific sport at a young age in hopes of gaining a college scholarship, playing professionally or both, it leads to injuries that may not have occurred otherwise.

“It’s turning into a nightmare of injuries that we’re losing control of,” said Dr. Christopher Ahmad, head team physician for the New York Yankees, chief of sports medicine and professor of orthopedic surgery at NYP/Columbia University Irving Medical Center.

Ahmad has made it one of his missions to educate fellow doctors, parents and their children on the dangers of specialization, and in his words “early professionalization.”

“They’re breaking their developing bodies,” he said. “It used to be that when I would see patients, I would see college and professional athletes with elbow pain. Now my office is filled much more with high school kids with elbow pain.”

Research is beginning to document a higher likelihood for single-sport athletes to get hurt.

A study involving more than 2,000 athletes ages 12-18 published in the American Journal of Sports Medicine in 2017 found connections between injuries and participants who specialized in one sport, with athletes who were considered highly specialized more likely to say they had an injury in the previous year than those with low specialization. The study also found that athletes who played their primary sport for more than eight months during the year were more likely to report they had experienced overuse injuries in their arms or legs.

Elbow pain in baseball, in particular, is something Ahmad has focused on. He did an informal poll with Yankees players who had elbow pain as a kid and found they were three times more likely to need Tommy John surgery later in life. Ahmad did research on youth baseball players in New Jersey and discovered that up to 80% play with pain in their elbows.

“When we asked these kids who do it, why play with elbow pain, they feel pressure to play through the pain,” he said. “When we consider sports specialization, they’re pressured by coaches in some way. Coaches tell them they need to dedicate themselves to a single sport and commit to a team. Often, they said, you can’t play on my team if you’re going to play other sports — in order to train them harder and win more games.”

The specialization trend is on the rise, with more kids looking to get a jump on the competition. They want to get seen by college coaches. They want to be great in one sport, rather than be good in a few different ones.

Christ the King girls basketball coach Bob Mackey has seen more girls devote their lives to the sport as early as the fifth grade. Matt Roventini, the baseball coach at powerhouse Poly Prep in Brooklyn, said he has freshmen who are talking about what colleges are looking at them before they even step onto the field as varsity athletes.

Oscar Osborne, a pitcher from Brooklyn who attends Poly Prep, suffered from Medial epicondyle apophysitis at the age of 13, which is also called “little league elbow.” It is a common injury among young pitchers whose bones have not yet stopped growing and can be attributed to overuse. Last year, he sprained the UCL ligament in his elbow, costing him his sophomore season. He admitted to pitching through pain, not because he was pressured to do it, but he felt it was important to keep pitching, as he attempted to work his way toward a college scholarship.

“It’s kind of like a job until you know where you’re going and you feel comfortable,” said Osborne, who is now healthy as he prepares for his junior season.

But playing only one sport does not necessarily translate to success down the road.

A 2017 study of 343 college athletes from a Midwest Division I University found the majority of players had not specialized on their sport throughout high school.

In addition, players may benefit from participating in several activities. Ahmad is a proponent of youth athletes playing multiple sports. He polled MLB draft prospects, asking if they had been multiple-sport athletes, and found that the healthiest ones didn’t focus on just one sport.

“I would say that the impact of getting injured [at a young age] is going to cut your career and compromise your career so drastically so that your dream may never come true,” Ahmad said. “You can make yourself have a better opportunity to get to the next level if you play a [second] sport.”

Joel Sanchez, the Preston High School volleyball coach who also runs the Legacy Volleyball Club travel team, points to several girls on his high school team who play three sports as an example of the benefits of variety. He said they are pictures of good health, while another athlete, who has played volleyball year-round her entire high school career, has been worn down.

“Her knees are shot. She can’t compete,” he said. “She can’t finish an entire set.”

Multi-sport advocates also cited possible psychological benefits.

Andy Kostel has coached high school soccer at Archbishop Molloy for 46 years, winning 15 city championships, and even he encourages his players not to focus too much on the sport he coaches. His sons played soccer and baseball.

He believes that playing more than one sport is good for young athletes mentally, pointing to all-time greats such as Michael Jordan, Hakeem Olajuwon and Bernie Williams who didn’t focus on just one sport as kids.

“That is so important because what is happening with quote-unquote academies, travel baseball at an early age and all that stuff is they lose their competitive edge because all they do is do the same thing all the time,” Kostel said. “Each season should be fresh.”

But many youth athletes — and their coaches and parents — still see specialization as an alluring path. A portion of that is because athletes have a love and passion for their specific sport and have little desire to play anything else. Others feel pressure to secure a college scholarship and eventually reach the pros.

Take Alex Apfel, for instance. She’s suffered myriad injuries. But if given the chance to do it all over again — to play multiple sports rather than spending her time only playing soccer — Apfel insisted she wouldn’t change a thing.

“She doesn’t stop,” her mother said. “It’s the one thing she always goes back to.”

Additional reporting by Joseph Staszewski and Elio Velez