Photo by Martin Sanchez (Unsplash)

Long-duration exercise is associated with fatigue, but what is fatigue?

Its definition depends on which fatigue model we use. Among them we find:

  • Cardiovascular/anaerobic fatigue

Fatigue comes when the cardiovascular system is unable to supply more oxygen or remove waste products from the working muscles.

  • Neuromuscular fatigue

The muscular response diminishes in response to the exercise electrical stimulus.

  • Muscle trauma fatigue

Fatigue is related to muscle damage.

  • Motivational fatigue

Associated to a lower interest in exercise performance.

  • Central governor fatigue

Central nervous system uses signals from muscles and organs to regulate exercise performance and therefore protect vital organs from injury or damage.


Marathon and the Rapoport´s energy model

A key moment associated with fatigue in a marathon is when hitting the wall, and Rapoport´s energy model tries to explain it. Thus, speed of glycogen stores (liver and muscles) depletion, linked to the fatigue, is associated with:

  1. runner’s aerobic capacity (or VO2max)
  2. density of muscle glycogen
  3. relative mass of leg musculature (larger legs = more room to store glycogen)

In this model a runner moving from A to B will need a certain amount of energy (calories) per kilometre and kilogram of body weight. 

Hitting the wall happens when running out of carbs and although they are the primary energy source, fats are also involved. Depending on running speed percentages of one source or another vary.

Substrate Usage (fat=RED or carbohydrates=BLUE) depending on Exercise Intensity (as % of VO2max) (from Rapoport 2010)

On a typical easy run, a runner uses 60% carbs and 40% fat. Carbs percentage increases with running speed, reaching 90% at intense paces.

The chances of hitting the wall in the Rapoport model are mathematically modelled and represented in the figure attached.

Considering that the average runner can run at an intensity between 60 and 85% of VO2max during a marathon and estimating the amount of carbs stored (shaded region indicates the range of energy storage capacity for a typical male runner, which is around 21.4% of body mass), would be possible to determine the distance where that runner would hit the wall.

Obviously, this is only a mathematical model subject to error (uncertainty around 5-10%), although it shows the importance of loading carbs adequately the days before a marathon.

Rapoport energy model



When carbs are out, then comes the wall
More carbs stored = less chances of hitting the wall


Good running and marathon(s) to you all!



Metabolic factors limiting performance in marathon runners. Benjamin Rapoport. PLoS Comput Biol. 2010 Oct 21;6(10): e1000960. doi: 10.1371/journal.pcbi.1000960.

How recreational marathon runners hit the wall: A large-scale data analysis of late-race pacing collapse in the marathon. Barry Smyth. PLoS One. 2021 May 19;16(5): e0251513. doi: 10.1371/journal.pone.0251513.


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