The unremarkable football career of Dr. Julian Bailes, who is now the chairman of the Pop Warner medical advisory board, ended quietly during his college days in the 1970s. He never sustained a concussion that he knew of, but he can recall a friend in high school who did. Confused and concussed, his friend started describing a car he had just bought; there was no new car, and Bailes found it hilarious. “We didn’t know any better,” he said.

Now, his son is a 13-year-old football player who understands the risks, Bailes said, and still wants to play, so Bailes allows him to.

For now, assuming the risk is Bailes’s choice, and his son’s. But the future of youth football may be determined by research that continues to redefine what the sport considers safe. On Wednesday, in an attempt to limit head injuries to young players, Pop Warner issued new rules that put restrictions on the amount of contact players can have in practice.

Jon Butler, the executive director of Pop Warner, said that research would continue to drive the organization’s rules changes as it tries to limit concussions. Researchers in the field liken Pop Warner — which has more than 285,000 children ages 5 to 15 in its leagues — to pioneers.