Marathon wall & glycogen depletion estimator
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Marathon wall estimator
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Method Guide
Marathon wall and glycogen depletion guide
Hitting the wall is the most feared experience in marathon running. This estimator models the mile-by-mile energy balance between glycogen stores, fat oxidation, mid-race fueling, and race pace to predict when depletion is likely to occur.
How the glycogen model works
Energy cost of running
Energy (kcal) = body weight (kg) x distance (km) x 1.04
Net metabolic cost of running is approximately 1 kcal per kg per km regardless of speed.
Substrate partitioning (crossover model)
Carb% = f(intensity %VO2max, training status)
At 70% VO2max, approximately 65-70% of energy comes from carbohydrate. At 85% VO2max, this rises to ~80-85%. Well-trained runners shift this curve toward more fat use at any intensity.
The estimator tracks glycogen depletion mile by mile, subtracting carbohydrate energy use and adding back mid-race carbohydrate intake. When glycogen drops below a functional threshold, the model flags wall risk.
Glycogen storage capacity
| Loading status | Glycogen (g/kg) | 70 kg runner (g) | Energy (kcal) |
|---|---|---|---|
| Not loaded | ~4.5 | ~315 | ~1,260 |
| Moderate | ~6 | ~420 | ~1,680 |
| Well-loaded | ~8 | ~560 | ~2,240 |
| Supercompensated | ~10.5 | ~735 | ~2,940 |
How to avoid the wall
- Carb load properly: Use a 3-day modified protocol to maximize starting glycogen.
- Run a conservative pace: Even 5 seconds/mile too fast significantly increases carb burn rate.
- Fuel during the race: 30-60g carbs per hour, starting at 30-45 minutes.
- Build aerobic base: High-mileage training improves fat oxidation and spares glycogen.
- Practice fueling: Train your gut to tolerate race-day carb intake.
Related tools
Use this with the Carb-Loading Planner, Race Strategy Calculator, and Pace Calculator.
FAQ
What causes "hitting the wall" in a marathon?
The wall is caused by critical depletion of muscle glycogen stores. When glycogen runs out, the body relies more on fat oxidation, which cannot sustain marathon pace. The result is a sudden, significant performance decline.
At what mile do most runners hit the wall?
Most commonly between miles 18-22 (29-35 km). The exact point depends on starting glycogen stores, pace intensity, mid-race fueling, training background, and individual fat oxidation capacity.
Can you prevent hitting the wall?
You can delay or prevent it by: (1) carb-loading before the race, (2) running at a sustainable pace (not too fast), (3) consuming 30-60g of carbs per hour during the race, and (4) building a high-mileage aerobic base that improves fat oxidation.
How does training volume affect the wall?
Higher training volume improves mitochondrial density and fat oxidation capacity. Runners with more aerobic training use proportionally more fat at any given intensity, preserving glycogen stores longer.
What is the crossover concept?
The crossover concept (Brooks & Mercier, 1994) describes how the body shifts between fat and carbohydrate as fuel sources depending on exercise intensity. At higher intensities, carbohydrate use increases while fat use decreases.
Does running slower prevent the wall?
Running slower shifts the fuel mix toward more fat and less carbohydrate, which preserves glycogen. However, you still need adequate glycogen stores and mid-race fueling for any marathon effort.
References
Balance of carbohydrate and lipid utilization during exercise: the crossover concept
Brooks, G.A. & Mercier, J. (1994). J Applied Physiology, 76(6), 2253-2261.
Metabolic factors limiting performance in marathon runners
Rapoport, B.I. (2010). PLoS Computational Biology, 6(10), e1000960.
Nutrition for endurance sports: marathon, triathlon, and road cycling
Jeukendrup, A.E. (2011). Journal of Sports Sciences, 29(S1), S91-S99.
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