A marathon is a 42.195 km test of the body’s ability to deliver oxygen, produce energy, control heat, and keep the brain focused for hours. Every stride depends on teamwork between the lungs, heart, blood, muscles, sweat glands, and nervous system. High school runners can use physiology to understand pacing, fueling, hydration, and recovery.
Knowing what happens inside the body helps turn training choices into smarter race-day decisions.
During endurance running, the heart pumps more blood to working muscles while breathing rate rises to bring in oxygen and remove carbon dioxide. Muscles use stored glycogen and fat to make ATP, the immediate energy molecule for contraction. If the pace is too fast, lactate and hydrogen ions build up faster than the body can clear them, which makes the effort feel harder.
Over a long race, heat stress, dehydration, electrolyte loss, and mental fatigue can limit performance even when the legs are still moving.
Understanding The Physiology of a Marathon Runner
VO2 max sets an upper ceiling, but marathon pace usually sits well below that ceiling. A runner with a high VO2 max can process a lot of oxygen during hard work, yet that alone does not guarantee a fast marathon. The important skill is economy.
Running economy means using less energy at a given speed. Good form, springy tendons, steady rhythm, suitable shoes, and practiced movement can all reduce the energy cost of each kilometre.
Two runners can have similar VO2 max values but need different amounts of oxygen to hold the same pace. Long training runs improve this skill because the brain, muscles, and connective tissues learn to repeat the motion efficiently.
Lactate is often misunderstood as a waste product that simply causes soreness. Muscles make lactate whenever they break down carbohydrate quickly. The heart and other muscles can use lactate as fuel, while the liver can help process it.
The problem comes when production exceeds removal for a sustained period. Hydrogen ions rise alongside rapid energy production and can interfere with muscle contraction. Breathing becomes much harder, legs feel heavy, and the pace becomes difficult to maintain.
Threshold training teaches a runner to hold a controlled hard effort without crossing too far into this unstable zone. A marathon requires restraint because even a small pace increase early in the race can create a large energy cost over many kilometres.
Glycogen is stored carbohydrate, mainly kept in muscles and the liver. Muscle glycogen supplies the muscle where it is stored. Liver glycogen helps keep blood glucose available for the brain and the rest of the body.
Fat stores contain far more total energy, but fat releases energy more slowly and needs plenty of oxygen. As pace rises, the body relies more heavily on carbohydrate. This explains why runners eat carbohydrate before and during a marathon.
Taking in small amounts regularly can preserve some stored glycogen and support blood glucose. The wall is not always a single event.
It can begin as fading concentration, a pace that suddenly feels impossible, chills, poor coordination, or strong cravings for food. Starting too fast makes it more likely because it burns carbohydrate at a faster rate.
Hydration is about maintaining circulation and cooling, not drinking as much as possible. Sweat removes heat only when it evaporates from the skin. In humid air, sweat may drip away instead, so cooling becomes less effective.
Blood is then needed both for muscles and for skin cooling, which puts extra strain on the heart. Sweat contains sodium, though the amount differs greatly between people. Very long events in hot conditions may require fluids with sodium, especially for heavy or salty sweaters.
Drinking excessive plain water can dangerously dilute blood sodium. Runners should practise their drinking and fueling plan during training, since the stomach must learn to tolerate food and fluid while running. Recovery needs carbohydrate to rebuild glycogen, protein to support repair, fluids to replace losses, and sleep to help the body adapt.
Key Facts
- VO2 max is the maximum rate at which the body can use oxygen during intense exercise, often measured in mL O2 per kg per min.
- Cardiac output = heart rate x stroke volume, and it rises sharply during running to deliver more oxygen to muscles.
- Lactate threshold is the exercise intensity where lactate begins to accumulate faster than it can be cleared.
- Stored muscle and liver glycogen are limited, and running out can cause the marathon wall.
- Heat balance depends on sweat evaporation, skin blood flow, air temperature, humidity, and running intensity.
- A useful energy estimate is Calories burned ≈ body mass in kg x distance in km for steady running.
Vocabulary
- VO2 max
- VO2 max is the highest rate at which the body can take in, transport, and use oxygen during hard exercise.
- Lactate threshold
- Lactate threshold is the running intensity at which lactate starts building up in the blood faster than the body can remove it.
- Glycogen
- Glycogen is the stored form of carbohydrate found mainly in muscles and the liver and used as a major fuel during running.
- Electrolytes
- Electrolytes are charged minerals such as sodium and potassium that help control fluid balance, nerve signals, and muscle contraction.
- Thermoregulation
- Thermoregulation is the body’s process of keeping internal temperature within a safe range.
Common Mistakes to Avoid
- Starting faster than goal pace, which pushes the body above a sustainable intensity too early and speeds up glycogen use and lactate buildup.
- Drinking only water for a long hot race, which may dilute blood sodium if intake is excessive and can increase the risk of hyponatremia.
- Ignoring carbohydrate fueling, which is wrong because glycogen stores are limited and low glycogen can cause a sudden drop in pace and focus.
- Assuming sweat rate is the same for everyone, which is wrong because sweat loss depends on body size, heat, humidity, pace, clothing, and acclimatization.
Practice Questions
- 1 A 60 kg runner completes a 42 km marathon. Using Calories burned ≈ body mass in kg x distance in km, estimate the runner’s energy use in Calories.
- 2 A runner has a heart rate of 170 beats per minute and a stroke volume of 120 mL per beat during a race. Calculate cardiac output in L per minute.
- 3 A runner feels good at mile 6 and speeds up well above planned pace, then slows sharply after mile 20. Explain using lactate threshold, glycogen depletion, and mental fatigue why this pacing strategy can fail.