As the average age of competitors in endurance sports rises, a spate of deaths during races or intense workouts highlights the risks of excessive strain on the heart through vigorous exercise in middle age.

The 40-to-60 age bracket — sometimes called “middle-aged men in Lycra,” or “mamils” — holds 32 percent of the membership in USA Triathlon, the sport’s official governing body. More fitness-conscious than previous generations, their numbers in competitive races are swelling, along with their risk of cardiac arrest. Triathlons, the most robust of endurance races, are also believed to be the most risky.

“People need to understand that they’re not necessarily gaining more health by doing more exercise,” said David Prior, a cardiologist and associate professor of medicine at Australia’s University of Melbourne. “The attributes to push through the barriers and push through the pain are common in competitive sport, but that’s also dangerous when it comes to ignoring warning signs.”

While the benefits of exercise are well known, researchers suspect that there may be a point at which exertion becomes dangerous, especially for middle-aged men.

Cardiac arrest, which occurs when the heart suddenly stops beating, can be caused by almost any heart condition, including abnormal heart rhythm and thickening heart muscle or arteries; such changes can occur silently as healthy people age. The risk of sudden cardiac arrest, which can be brought on with physical stress, increases with age, and men are two to three times as likely to suffer from it as are women, according to the National Institutes of Health.

Michael McClintock, 55, a bank executive who died June 2 at his home in Larchmont, N.Y., was a skier, biker and golfer. Last September, he completed his first Olympic-length triathlon, taking fewer than four hours to swim 0.9 miles, cycle 25 miles and run 6.2 miles.

While McClintock’s death can’t be directly linked to the race, USA Triathlon has noted an increase in race-related fatalities, with the highest number occurring in the 40-to-49 age group.

SWIMMING MOST RISKY

The death rate for triathlons is about twice that for marathons because of the increased intensity of the competition and the initial swimming leg of the events, according to a 2012 study in the journal Mayo Clinic Proceedings.

“The swim seems to be a particularly dangerous time,” said marathoner Andre La Gerche, a cardiologist at Melbourne’s St. Vincent’s Hospital. “Paradoxically, in the marathon, it’s the opposite: It’s the last mile of the event where the vast majority of fatalities occur.” Researchers speculate that sprinting to the finish produces a rush of adrenaline that may trigger an abnormal rhythm in runners with susceptible hearts.

The swim leg of the triathlon, often held in open water, can be “extraordinarily stressful,” said La Gerche, who has competed in more than 100 triathlons. “You have people climbing all over you. Sometimes you’re fighting to breathe, and that’s not something the body is used to.”

Open-water racing triggers a clash of two mechanisms of the involuntary nervous system, according to researchers at England’s University of Portsmouth. A “fight or flight” response activated by physical exertion, cold water temperature or anxiety tries to speed up the heart rate and causes hyperventilation; this occurs just as the body tries to slow the heart rate to conserve oxygen in response to facial wetting, water entering the mouth, nose and throat, and extended breath-holding, the scientists said.