The most common reason a marathon runner collapses is surprisingly simple: they stop running. During a race, your leg muscles act as a pump, rhythmically squeezing veins to push blood back up to your heart. The moment you cross the finish line and stop moving, that pump shuts off. Blood pools in the dilated vessels of your legs, your brain temporarily loses adequate blood flow, and you go down. This is called exercise-associated collapse, and it accounts for the vast majority of finish-line incidents. But it’s not the only explanation. Depending on the circumstances, the biology behind a collapse can involve overheating, dangerously low sodium, depleted fuel stores, muscle breakdown, or, rarely, a cardiac event.
Blood Pooling at the Finish Line
Your venous system holds roughly 70% of your total blood volume, about 3.5 liters in an adult. During running, the contracting muscles in your calves and thighs squeeze those veins like a bellows, driving blood upward against gravity and back to the heart. Your blood vessels also dilate widely during prolonged exercise to help dissipate heat through the skin. As long as you keep moving, the muscle pump compensates for this dilation and maintains enough blood pressure to keep your brain supplied.
When you stop abruptly, the pump stops but the dilation doesn’t reverse immediately. Blood settles into your legs and the large vessels of your abdomen. Venous return to the heart drops, cardiac output falls, and blood pressure plummets. The result is transient postural hypotension, essentially a standing faint. It looks dramatic, but it’s the most benign reason a runner collapses. Medical teams treat it by laying the runner down and elevating their legs, which uses gravity to redirect that pooled blood back toward the heart. Most runners recover within minutes with this positioning and some oral fluids, no IV or advanced treatment needed.
Running Out of Fuel
Your body stores carbohydrate as glycogen: roughly 400 grams in your muscles, 100 grams in your liver, and about 5 grams circulating in your blood as glucose. At marathon pace, those stores can be exhausted well before the finish line. When liver glycogen runs out, blood sugar drops. Researchers observed this as far back as the 1923 Boston Marathon, where finishers showed blood glucose levels below the normal threshold.
Low blood sugar starves the brain of its preferred fuel. This is where the famous “hitting the wall” comes from: your legs feel like concrete, your thinking gets foggy, coordination deteriorates, and in severe cases you can lose consciousness. Faster runners burn through glycogen more quickly per minute, which is why elite athletes are meticulous about mid-race carbohydrate intake. Without it, the central nervous system essentially dims the lights to conserve what little glucose remains.
Overheating and Heat Stroke
During a marathon, your muscles generate enormous heat. Your body cools itself by sweating and by shunting blood to the skin, but on warm or humid days, heat production can outpace heat loss. When core body temperature climbs above about 104°F (40°C), you enter dangerous territory. Exertional heat stroke is clinically defined by a core temperature above 105°F (40.5°C) combined with signs of brain dysfunction: confusion, irrational behavior, irritability, seizures, or loss of consciousness.
At these temperatures, proteins in your cells begin to denature and your organs start to suffer direct thermal damage. The brain is particularly vulnerable, which is why altered mental status is the hallmark of heat stroke rather than simply feeling hot and tired. Heat stroke is a medical emergency. Unlike benign exercise-associated collapse, it requires rapid whole-body cooling, not just leg elevation. The distinction matters: a runner who collapses and is confused, combative, or unresponsive needs their temperature checked immediately.
Sodium Drops Too Low
Exercise-associated hyponatremia occurs when blood sodium concentration falls below 135 mmol/L during or after a race. It typically happens not because a runner loses too much sodium in sweat, but because they drink too much plain water, diluting the sodium already in their blood. Sweat sodium concentrations vary widely between individuals. Some runners lose less than 30 mmol of sodium per liter of sweat, while “salty sweaters” lose 60 mmol or more per liter. But even salty sweaters in studies have not developed hyponatremia from sweat losses alone. Overdrinking is the primary culprit.
When blood sodium drops below about 120 mmol/L, the concentration difference between blood and brain tissue causes water to flow across the blood-brain barrier into the brain, producing cerebral edema, or swelling. Symptoms at this stage include severe headache, confusion, coordination problems, and unconsciousness. If sodium falls to 110 to 115 mmol/L, respiratory failure and death become real possibilities. This is why current guidance for marathon runners emphasizes drinking to thirst rather than forcing fluids on a fixed schedule.
Muscle Breakdown and Kidney Stress
Marathon running causes significant muscle damage. One measurable sign is creatine kinase (CK), an enzyme that leaks from damaged muscle cells into the blood. In a study of marathon finishers, average CK levels rose from 161 U/L before the race to 3,424 U/L within 24 hours after. Faster runners showed even higher levels: those finishing under 3.5 hours averaged 4,433 U/L compared to 1,432 U/L for slower finishers. The harder the effort, the more muscle tissue breaks down.
When CK levels exceed roughly 5,000 U/L, the condition qualifies as rhabdomyolysis, and the breakdown products from damaged muscle can clog the kidneys’ filtering system, potentially causing acute kidney failure. The classic warning sign is dark, tea-colored urine, though only about 3.6% of rhabdomyolysis patients actually report it. Rhabdomyolysis doesn’t always cause a runner to collapse mid-race, but it can contribute to extreme fatigue, nausea, and disorientation that lead to a fall, and it becomes a serious concern in the hours after the race if untreated.
Cardiac Arrest: Rare but Real
Sudden cardiac arrest during a marathon is the most feared cause of collapse, but it is uncommon. Among 29.3 million race finishers tracked between 2010 and 2023, 176 cardiac arrests occurred, a rate of 0.54 per 100,000 participants. Cardiac death was even rarer, at 0.20 per 100,000, a rate that has actually declined over the past two decades as race medical teams have become better equipped with defibrillators and rapid-response protocols.
In younger runners, cardiac arrest usually stems from inherited structural heart conditions, such as a thickened heart muscle, that may have gone undetected. In runners over 40, the more common cause is coronary artery disease, where a plaque in a heart vessel ruptures under the stress of exertion. Unlike the other causes of collapse on this list, cardiac arrest means the heart has stopped producing an effective rhythm. The runner will be unresponsive with no pulse, and survival depends on bystander CPR and rapid defibrillation.
How These Causes Look Different
The biology behind each type of collapse produces different patterns that help medical teams respond correctly. A runner who collapses right at or just past the finish line, is conscious, and improves when lying down with legs elevated almost certainly has benign blood pooling. A runner who is confused or combative on a hot day points toward heat stroke and needs a temperature check. Collapse with seizures or progressive confusion over hours, especially in a slower runner who drank heavily during the race, suggests dangerously low sodium. Sudden, unresponsive collapse with no pulse is cardiac arrest until proven otherwise.
Most marathon collapses fall into the first category. The body’s cardiovascular reflexes simply can’t adjust fast enough when the muscle pump stops. It’s a design quirk of human physiology: the same system that keeps you running efficiently for hours needs a few minutes of walking to wind down gracefully. Skipping that cooldown, sprinting through the finish and then standing still, is the most reliable recipe for ending up on the ground.