Inside the knee is a diagonal band of fibrous tissue, roughly the size of pinky finger. When it snaps, the course of an NFL season changes.

On football injury reports, few words are as dreaded as “anterior cruciate ligament.” ACL tears once ended careers. Even today, they often signal lost seasons.

“It’s certainly a big cluster,” said Dr. Bert Mandelbaum, a Santa Monica-based orthopedic surgeon and ACL injury prevention researcher at the Kerlan-Jobe Institute. “There’s no question about it being a big cluster when you have that many ACL injuries on any one professional team. It’s alarming.”

An important caveat: Although Mandelbaum has worked closely with high-level athletes, including the Galaxy and the U.S. Men’s National soccer team, he has not directly examined any of the injured Chargers. Barring closer analysis on his part, he could not say whether or not the Chargers’ rate of ACL tears indicated simple bad luck or an underlying systemic problem.

But Mandelbaum also has spent nearly two decades studying how to prevent and reduce ACL injuries, a quest that began when he and other doctors saw a spike in knee injuries among female teenage athletes in Southern California. They realized that when the athletes were jumping, landing or decelerating, deficiencies in their hip caused the upper legs to turn in, excessive strain on the ACL.

In response, he developed the PEP program, which consists of warm-up and strengthening exercises, plyometrics and stretches in order to promote better posture and control.

This approach, Mandelbaum believes, could also benefit professional football players. Over the past three seasons, he worked with team doctors, including the 49ers’ Timothy McAdams and the Giants’ Scott Rodeo, to examine film of ACL injuries. In doing so, they found 68 instances of non-contact ACL injuries.