Why Do Marathon Runners Collapse?

Here, we’ll explore why do marathon runners collapse—or even suffer cardiac arrest—during marathons and other endurance events. 

Running A Marathon: What the Body Endures

For both aspiring and seasoned triathlon, marathon, and half marathon athletes, it’s hard to ignore the thought of becoming one of the runners who collapse on the sidelines, needing medical attention. Most runners who have participated in an endurance event have witnessed other runners needing to visit the medical tent.

It can be scary to see runners struggling, and it’s smart to learn what causes a runner to collapse during long-distance races. There are several reasons endurance athletes may experience an inability to continue during endurance races.

Here, we’ll explore some of the most common issues that result in a collapsed athlete, including cardiac arrest and heart disease, hyponatremia (electrolyte imbalance), heatstroke, hypertrophic cardiomyopathy (underlying, typically unknown cardiac abnormalities), as well as postural hypotension, which can cause serious health issues or death after the endurance event is complete. 

Cardiac Arrest And Underlying Heart Disease

Cardiac arrest doesn’t happen often (less than one percent of half marathon and marathon endurance athletes experience cardiac arrest), but when it does happen, it’s often deadly.

Cardiac arrest occurs when there’s a misfiring in the electrical signals that keep the heart beating properly, resulting in a loss of blood flow to the heart. This misfiring (also known as cardiac arrhythmia) causes the heart to stop beating. 

About 70% of the athletes who experience cardiac arrest pass away due to cardiac death. Sudden cardiac arrest can be related to underlying heart disease or previously unknown heart defects. Some runners who survive cardiac arrest are eventually able to return to their normal running schedule without health issues.

Electrolyte Imbalance

Taking on a new challenge — such as a marathon or half marathon — can surprise even a seasoned endurance athlete, no matter how high their level of cardio fitness. Changes in body temperature, changes in mental status, and fluctuating sodium levels due to new levels of physical exertion may cause some runners to overhydrate. 

This does far more than just create an uncomfortable bloated feeling — it can result in hyponatremia, a serious condition in which the sodium levels in the blood are dangerously low. If a collapsed runner is treated quickly (without intravenous fluids), it’s possible that their sodium levels can return to normal. The incidence of hyponatremia is especially high among female ultramarathon runners. 

Heat Stroke

Most runners have been there: it’s time for a huge race that marks the end of months of training, but running conditions aren’t ideal. When you run, your heat production multiplies by twenty — and when you’re running in conditions that are hotter than what you’re used to, the result can be disastrous. 

Interestingly, a study by Dr. Timothy Noakes revealed that marathoners are less likely to suffer from heat stroke than 5K runners. In a long-distance race, the brain communicates with the body when temperatures are rising too high, too fast, and forces the body to slow down.

The problem: this communication takes time. In a shorter race, runners who are psyched to get to the finish line may work faster than their brain’s warning systems can handle, increasing the likelihood of heatstroke. 

Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy is a genetic disorder that causes an unusual thickness of the left wall of the heart. Intense physical exercise can increase the thickness of the walls of the heart in a person without this condition.

Athletes who have hypertrophic cardiomyopathy are at increased risk for sudden cardiac death. If you’ve had a family member pass away suddenly, it’s possible that they had undiagnosed hypertrophic cardiomyopathy, and it’s more likely that the same could happen to you. 

Often, an athlete’s hypertrophic cardiomyopathy is not known until after they pass away due to a sudden cardiovascular event. If an athlete knows that they have the condition, or have a family history of the condition, it’s important to talk with a physician about how to exercise and compete in a way that makes sense for their health. 

After The Event: Postural Hypotension

Many runners feel that they’re in the clear after they cross the finish line, but this isn’t necessarily the case. Postural hypotension is a dangerous condition that can occur after the race is over when finishers are ready to recover. 

Postural hypotension is marked by the pooling of blood in the legs. Treatment is simple: runners simply need to lie down with their head and pelvis elevated. The biggest concern with postural hypotension isn’t death (the body will faint long before serious damage occurs) — it’s falling and getting hurt. 

Staying Safe

If you’re training for a new distance or making serious changes to your training routine, it’s smart to talk with your doctor to ensure that you’re making the right moves to lower your risk of exercise-associated collapse. 

While many common causes (such as running during excessive heat or overhydrating) occur on race day, always been on the lookout for over-exertional symptoms during training. Let someone know where you’ll be when you’re training, keep an eye on your heart rate and hydration levels, and stop running if something feels off.

If you start to feel not quite like yourself during a race, be sure to alert nearby sports medicine personnel. 

Final Word on Why Do Marathon Runners Collapse

There are many reasons for runner collapse, ranging from benign (postural hypotension) to more severe issues, such as cardiac arrest. If you experience a running-related collapse, be sure to talk with a sports medicine specialist before you resume training. 

You might also be interested in our explainer on why do runners get shin splints.

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