(Photo: Courtesy Keen)
Correction: A previous version of this story incorrectly stated that Jeff Browning suffered a heart attack on August 24, 10 miles from the finish at TDS. He suffered a cardiac arrest on August 25, 55 miles into the race.
What You Need to Know
On August 25, storied ultrarunner Jeff “Bronco Billy” Browning was 57 miles (91K) into TDS, a notoriously demanding 145K race held during the UTMB Mont-Blanc festival in Chamonix, France, when he suffered a cardiac arrest. Fortunately, the 55-year-old, who has won over 30 100-mile races, was at an aid station when he collapsed. A medical crew with a defibrillator leapt into action, providing lifesaving care, and Browning was helivaced to a hospital where he underwent an emergency procedure to place three stents.
Two months earlier, on June 16, retired track star Jenny Simpson had just finished pacing a group of girls at a Sir Walter Running pop-up mile event in Raleigh, North Carolina, when she suffered a cardiac arrest. Were it not for the quick-acting bystanders and first responders who performed CPR and used an Automated External Defibrillator (AED) on Simpson, the 2011 world champion and 2016 Olympic bronze medalist in the 1500 meters would likely have died.
Simpson, 39, suffered a second cardiac arrest after being transported to UNC Rex Hospital. She was put on life support and transferred to Duke University Hospital, where she was placed on a ventilator for approximately four days.
As trail runners, many of us assume that our fitness protects us from heart disease, which makes the news about Browning so soon after Simpson’s own cardiac event understandably confusing and alarming. To get to the bottom of the relationship between running and our hearts, we spoke with Simpson’s cardiac electrophysiologist, Albert Sun, MD, Director of the Ventricular Arrhythmias Program and the Inherited Arrhythmias Program at Duke University Medical Center, and Bradley Lander, MD, Director of the Sports Cardiology Center and the Hypertrophic Cardiomyopathy Center, University Hospitals Harrington Heart and Vascular Institute.
“Trail runners and endurance athletes need to understand that your sport does not make you immune to these heart conditions or cardiac events,” Lander tells Trail Runner.
Before we dive in, it helps to understand what a cardiac event is (hint: it’s not the kind of event you want an invite too). Also known as a cardiovascular event, it can be any sudden disruption of the heart’s blood flow or normal function. While heart failure, heart attacks, and sudden cardiac arrest all fall under the umbrella of cardiac events, each is distinctly different.
Heart failure occurs when the heart muscle doesn’t pump as much blood as the body needs. While it may come on suddenly, it’s typically a long-term, chronic condition.
A heart attack is usually caused by a lack of blood flow to the heart because of blocked arteries, a tightening of the arteries, or a tear of the arteries. If blocked arteries are not reopened quickly, the part of the heart fed by those arteries will begin to die, which is why it was so vital that Browning, who suffered from a serious heart attack caused by blocked arteries, underwent an emergency procedure during which three stents were inserted to open the blocked arteries and restore healthy blood flow to his heart.
While a heart attack is a circulation problem, meaning the heart usually doesn’t stop beating, a cardiac arrest is an electrical problem that occurs when there’s a disruption of the heart’s electrical conduction system, and the heart suddenly and completely stops beating. When this happens, the necessary blood and oxygen are no longer pumped to the brain, lungs, or other organs. This can cause a person to lose consciousness, a pulse, and possibly their life if the heart is not restarted within minutes.
Simpson’s cardiac arrest was determined to be caused by arrhythmogenic right ventricular cardiomyopathy (ARVC), a rare genetic heart condition that she unknowingly had her entire life. Also known as arrhythmogenic right ventricular dysplasia and arrhythmogenic cardiomyopathy, ARVC replaces healthy heart muscle in the right ventricle with fibrous and fatty scar tissue, which cannot conduct electrical signals properly. In Simpson’s case, the scar tissue within her heart interrupted its electrical system, triggering dangerous rhythms that caused her heart to stop.
Don’t hang up your hydration pack for the sake of your heart just yet. For the majority of people, running does not trigger cardiac events.
The Centers for Disease Control and Prevention reports that 805,000 people have a heart attack every year in the U.S. Meanwhile, about 350,000 out-of-hospital cardiac arrests occur annually in the U.S., according to the American Heart Association. That’s one in every 980 people.
While it’s an apples to oranges comparison with too many confounding variables to count, these rates are remarkably higher than what runners suffer doing races, according to a 2025 JAMA study examining 29.3 million marathoners and half marathoners from 2010-2023. In those participants, there were 176 mid-race cardiac arrests, or roughly one cardiac arrest per 167,000 racers.
“Running any distance is still incredibly healthy. In fact, running has the potential to lower your risk of having a cardiac event by helping to prevent heart disease,” Sun says.
Distance running can lower blood pressure, improve cholesterol levels, and reduce the risk of diabetes and obesity, Sun says. And trail running has the added benefit of improving balance and coordination as you traverse an array of changing elevations and terrain.
“Let’s say you decide to train to run for 24 hours or more at a reasonably fast pace. You’re asking your body to give you a constant, high level of cardiac output during your training, and these performance gains come from real physiologic changes,” Sun says, explaining that the heart adapts to intense exercise over time. “Like any muscle, your heart responds to training by getting stronger and more efficient, so it can better deliver blood and oxygen to your muscles over long hours of running.”
However, trail and ultrarunning cannot remove blockages or reverse the cumulative effect of decades of structural damage caused by chronic high blood pressure and high cholesterol. And if you are genetically predisposed to heart disease, that’s not something you can outrun.
Don’t freak out, but… long-distance running can damage the heart in some cases.
In a study of 46 athletes competing in the Penyagolosa Trails CSP115 race, a 74-mile ultramarathon with a total elevation gain of 17,850 feet, researchers found that the event induced an acute release of cardiac damage biomarkers jointly with cardiac autonomic modulation (the medical term for how your brain unconsciously tunes and balances your heartbeat). The study highlighted the fact that the faster the runner, the greater the increase in cardiac damage biomarkers, which provide evidence of heart muscle injury or stress.
“Because of the physical and metabolic stress that long-term, high-intensity endurance running puts on the heart’s blood vessels, there can be a slightly higher risk for coronary artery calcification, a build-up of calcium deposits within the walls of the arteries that supply blood to your heart,” says Lander, who is also an assistant professor at Case Western Reserve University School of Medicine. “Normally, a higher coronary calcium score increases the risk for heart attacks, but the risk-benefit of long-term exercise favors a lower cardiovascular risk for people who exercise regularly.”
In other words, running can remodel the heart in less and more beneficial ways.
For trail runners who have an underlying heart condition, such as coronary artery disease, a cardiomyopathy, or a rare inherited or congenital heart condition, the stress of pushing your body to the extreme in distance, pace, or both can elevate the risk of sudden cardiac arrest.
“That’s why we hear about runners experiencing a cardiac event at the end of or immediately after a race,” Sun says. “During those periods of high exertion, runners are often putting increased stress on their heart. So if they have an underlying heart condition, there’s a higher risk of experiencing a cardiac event. That said, this is still extraordinarily rare,” he continues, adding that the best way to lower this risk is to see your primary care physician regularly so any underlying conditions can be appropriately evaluated and treated.
As endurance athletes, we get accustomed to pushing through pain to go the distance. But when it comes to heart health, some symptoms should, quite literally, stop us in our tracks. These can include a heavy pounding of the heart, pain or tightness in the chest, pain that spreads from the middle of the chest to the arm, back, or jaw, abnormal heart rhythms, shortness of breath, unusual fatigue, faintness, lightheadedness, nausea, sudden cold sweats, or loss of consciousness.
“If you have any of those symptoms during a run or race, stop and rest. If symptoms continue, or if you lose consciousness, call emergency services immediately,” Sun says. “Even if the symptoms subside, report them to your physician so you can be evaluated for heart disease or another underlying heart condition.”
If you notice your heart suddenly racing and it’s out of proportion to your effort, or you feel a flip-flopping or irregular sensation in your chest, this should also be reported as it may be a symptom of an arrhythmia.
While the chances you’ll suffer a cardiac event or low, it’s much more likely you will witness one. The chances that a runner (or anyone) will survive a cardiac arrest specifically triple when CPR is performed immediately. If you’re on a training run or competing and suspect that another runner is having a heart attack, call 911 or your emergency response number immediately.
If you are concerned that another runner has gone into cardiac arrest, call 911 and check for responsiveness. Then begin CPR, and use an AED if one is available, as it sends an electric shock to the heart to restore its normal rhythm. Nine out of ten cardiac arrest victims who receive a shock in the first minute will survive.
The rate of cardiac arrests hasn’t necessarily changed much over the years and is still very rare: As noted above, less than 1 in every 100,000 marathon runners will experience sudden cardiac arrest. But survival has dramatically improved, with cardiac deaths decreasing from 0.39 per 100,000 to 0.20 per 100,000 participants.
“That’s primarily because of increased race preparedness, early activation of 911 emergency services, prompt CPR, and access to an AED,” Sun says. “Running can be super safe as long as you have an emergency action plan. Learning CPR is an extremely important part of that. So is carrying a cell phone, especially if you’re running on a remote trail, so you can call for help.”
Browning is back home in Flagstaff, Arizona, with his family, recovering from two procedures, a surgery, and nine days in France’s Hôpital Michallon. In an emotional Instagram post on September 3, he shared that “there is a path forward and a path to [me] getting stronger and back on the trails.”
Simpson has also returned to her home in Boulder with her husband and dogs. On August 20, she shared that, although running didn’t cause her cardiac arrest, it revealed a diagnosis that makes it too dangerous for her to return to the sport she loves.
“Jenny has a heart condition where more and higher stress can potentially lead to more heart damage, and that’s why, for the time being and potentially indefinitely, we are limiting her physical exertion,” Sun says. “In Jeff’s case of blockages in the arteries, stents restored adequate amounts of blood flow and oxygen to the heart muscle, which is why he can build back up to running eventually.”
Running is a paradox. It can be hard on your heart while simultaneously improving your chances of surviving a cardiac event. Regardless of how fit (and fast) you are, the smartest approach is to prioritize regular screenings for adult onset conditions and heart disease, and self-monitor your own heart health.
“If you listen to your body, you can run without fear,” Sun says. “Ultrarunners are remarkably amazing examples of what the human body is capable of.”