WHY DO MARATHONERS FADE AT 35–40 KM
COACH’S KAMAU WAWERU CORNER:
WHY DO MARATHONERS FADE AT 35–40 KM EVEN WHEN HEART RATE LOOKS FINE?
One of the most frustrating marathon experiences is this: -You are running comfortably at your target pace. -Your breathing is controlled. -Your heart rate is still where it should be.
Then, somewhere around 35–40 km, the legs suddenly stop responding.
Pace drops. The effort feels much harder. The final kilometres become a survival exercise.
So what is happening?
THE SCIENCE: A marathon is not simply a cardiovascular race. It is also a test of muscular endurance, glycogen availability, running economy and neuromuscular fatigue.
a) Glycogen depletion: At marathon intensity, carbohydrates provide a major source of energy. As muscle glycogen becomes progressively depleted, maintaining the same pace becomes increasingly difficult. That is why fueling should not start when you feel tired. By then, the problem may already be developing.
b) Peripheral muscular fatigue: Your heart and lungs may still be capable of delivering oxygen, but the muscles involved in running can become fatigued. The result?
- Cardiovascular system: still working well.
- Muscles: losing the ability to produce the same force efficiently.
This is one reason heart rate can remain relatively stable while pace deteriorates.
c) Reduced running economy: As the legs fatigue, stride mechanics can change. Ground-contact time may increase, coordination can deteriorate and more energy is required to maintain the same speed. The pace that felt economical at 25 km can become extremely expensive at 38 km.
d) Neuromuscular fatigue: Marathon running requires thousands of repetitive contractions. By the final kilometres, the nervous system and muscles are dealing with accumulated fatigue. This is why marathon preparation must go beyond simply developing a big aerobic engine.
SO HOW DO WE FIX IT?
1. Build aerobic volume progressively: Consistent mileage develops mitochondrial capacity, capillarisation and the ability to use oxygen efficiently. But volume must be individualised. More kilometres are not automatically better. 2. Develop the lactate threshold: Threshold training improves the ability to sustain a high percentage of aerobic capacity without accumulating excessive metabolic stress. This allows marathon pace to become relatively economical rather than excessively demanding. 3. Train marathon-specific durability: Long runs should sometimes include controlled periods at or around marathon pace. The objective is not simply to run 35–40 km. The objective is to teach the body to maintain efficient mechanics when already fatigued. 4. Practise fueling: Don't make race day your first fueling experiment. Train the gut just as you train the legs. Practise carbohydrate intake during long runs and establish a strategy you can tolerate and execute consistently. 5. Respect recovery: High mileage and quality sessions only work when the body has enough time to adapt.
Training creates the stimulus. Recovery creates the adaptation.
THE BIG LESSON;
A fast marathon requires more than a strong heart and high VO₂ max. You need the ability to keep producing efficient running mechanics after 30, 35 and 40 km of accumulated fatigue. That's why the marathon is ultimately a test of aerobic fitness + running economy + fueling + muscular durability + discipline.
Don't just train to run fast for 30 km.
Train your body to keep running efficiently when the marathon starts to hurt.
STAY HARD.
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