RYUSEI was just 11 years old. He was sitting in a chair and listening to a doctor, who stood in front of a computer screen showing an image of an elbow ligament damaged from repeatedly throwing a baseball. Ryusei smiled. He didn’t seem to realize the ligament was his.

Twice a week, the doctor, Naotaka Mamizuka, invites youth baseball players from across Japan to have their arms examined at the hospital where he works, about two hours from Tokyo. A spinal surgeon by trade, Dr. Mamizuka developed an interest in helping the children who are chewed up and spit out by Japan’s unforgiving youth baseball machine, which encourages year-round training and, during its national high school baseball tournaments, lionizes 200-pitch games.

The day I visited, Ryusei was one of 20 patients, some as young as 9. Dr. Mamizuka diagnosed injuries in 19 of them. Almost all suffered damage to the ulnar collateral ligament, a triangular band of tissue that connects the upper and lower arms. In some cases the ligament was torn, requiring the procedure known as Tommy John surgery, common among pitchers in Major League Baseball. But the ligament itself didn’t tear in most of the younger children; instead, it pulled off a piece of still-unhardened bone, leaving a fracture in the elbow.

“I say to parents: This is not good,” Dr. Mamizuka told me that day. “Bad training. Bad pitching. Bad coaching.”