“The nerve supply to fast-twitch muscle fibers dies off, and a fiber that once had fast-twitch capabilities is converted to a slow-twitch fiber,” he said. “While you still have a functioning muscle fiber, the fiber is now an endurance fiber.”

Since the fast-twitch fiber is designed to produce strength and power, people naturally lose both as they age. That affects the body’s ability to maintain a fast pace when tired — unless a runner incorporates speed work and strength training into preparation for races. That is how Meb Keflezighi, who is 43 and retired but toying with a comeback, set a personal record at 38 in 2014 and qualified for the United States Olympic team two years later, beating out plenty of runners a decade younger.

In addition to running anywhere from 85 to 130 miles each week at varying distance intervals and speeds as they prepared for New York, Abdirahman, Lagat and Barrios spent several afternoons each week in the gym to maintain the strength that age is naturally sapping from them.

In fact, those workouts have become increasingly common among lay runners, too, which might help explain why more serious runners far below the elite level may be getting faster, too.

Earlier this year, the Boston Athletic Association announced that it needed to lower the qualifying standards for runners trying to qualify for the Boston Marathon. Boston is the rare marathon that requires runners to attain a qualifying time to get a place in the field (unless they take one of the spots reserved for those who raise money for charity).

In recent years, the race’s organizers have been unable to accept all runners who have met the qualifying standards, which vary by age. In 2017, runners had to be 2 minutes 9 seconds below the qualifying standard. In 2018, they had to be 3:23 lower. For 2019, they will have to have run 4:52 lower. Because of increasing applications and an apparent spike in the number of people running qualifying times in 2018, race organizers lowered qualifying standards for all age groups by five minutes this year, meaning their analysis suggested it wasn’t just younger runners who were getting faster but older ones, too.