One of the ultimate tests of fitness for more than a century has been the triathlon, which takes swimming, bicycling and running, rolling it into one competition. There have been people that participate in triathlons for fun ever since the sport’s creation, while many try to win at the highest level. Since 2000, that level has been the Olympics, as the triathlon was recognized as an official sport.

The standard international competition for the triathlon requires just under one mile of swimming, followed by 25 miles of bicycling and 6.2 miles of running. That’s enough to test your endurance, and you have to be in terrific shape to get through the entire race. Many that have tried and found out they aren’t quite prepared usually bow out in the middle of the race, but researchers have found that some participants are pushing too hard, and it’s costing them their lives at a surprising rate.

Dr. Kevin Harris is a cardiologist at the Minneapolis Heart Institute Foundation at Abbott-Northwestern Hospital, and he led a study to find the death rate of triathlon competitors. What he found is that since 1972 when the triathlon became a globally recognized sport with an increase in competitors, there have been more than 9 million competitors, with 1.74 out of each 100,000 of them dying suddenly during the triathlon.

Other long distance competitions such as marathons (26.2 miles) didn’t have nearly as high of a death rate according to the study. Harris found that middle-aged triathletes were at the highest risk, especially in just the first mile of the race. “The majority of deaths occur in the swim portion of the triathlon, which is the first portion of the race,” he said.

Harris added that while swimming, “the athletes likely experience an adrenaline surge as they enter the water and are competing in close proximity to other athletes, and in some cases with environmental conditions that are difficult to prepare for. We don’t understand the exact cause of death in each athlete, and some swim deaths may be related to drowning.”

The study specifically looked at sudden deaths that happened in the United States between 1985 and 2016 during officially registered triathlons. There were a total of 122 deaths during these competitions, with 13 of them happening via cardiac arrest. What was even more surprising is that the deaths weren’t typically occurring during longer triathlons, but rather shorter ones than international competition.

Two-thirds of triathlon deaths happened during swimming, while those that survived a smaller cardiac arrest and made it later in the triathlon ultimately had another episode while getting to the final running portion of the triathlon. Overall, the triathlon has been deemed as one of the deadlier sports out there because of the study, even if the numbers might seem small at 1.74 deaths of 100,000 participants. The chances of a healthy athlete suddenly dying during exercise overall is just 0.5 out of 100,000, meaning that 1,000 times more people are dying during triathlons.

Younger people don’t see too much risk of sudden death during a triathlon. For those under 30 years old, the female death rate per 100,000 was almost zero, while men were just over 2.0 for every 100,000. When you get above 60 years old, though, men were dying at an alarming rate of 20 deaths per 100,000 competitors. Researchers found that undocumented heart disease was the biggest culprit for these deaths.

Once you hit 40 years old, the risks of sudden death during a triathlon start to skyrocket, though the death rate for women is significantly lower than it is for men. No matter your gender, the researchers suggest that those who plan on participating in a triathlon should speak with a cardiologist, especially if you’re over 40. The average age of people who died suddenly was 47 years old, with well over three quarters of victims being men.

Dr. Reginald Ho of Sidney Kimmel Medical College chimed in, saying triathletes “should see their physician regularly for health checks that include a complete history and physical examination. Any heart related symptoms (such as shortness of breath or chest tightness) should be reported to their doctor.” The signs for a potential death during a triathlon can be spotted while training.

Researcher Hannah Arem says that these numbers overall shouldn’t discourage people from swimming, bicycling or running, though. “For the majority of individuals, engaging in regular exercise will yield more benefit than harm,” she said. These three exercises are among the best you can do for cardiovascular health, but just make sure you aren’t pushing too hard, and always keeping up with regular check-ups at the doctor’s office, researchers say.