9. DURABILITY, CARBS INGESTION AND MUSCLE GLYCOGEN

Photo by Adrien Brun on Unsplash

‘I tried to eat as much as possible but nothing went to my legs. Everything stayed in my stomach. I was really empty after 3h30m, at the bottom of the Col de la Loze climb. Had I not had such great support in the climb, I may have lost the podium. I kept fighting with Marc [Soler]. I’m grateful to him, and to all my teammates.’

- Tadej Pogacar, after Stage 17 of Tour de France 2023 -

Durability is the capacity to decrease as less as possible performance during prolonged exercise. In previous posts we saw that:

1. Durability is not related to traditional physiological parameters (VO2max and thresholds) at rested state. So, a higher VO2max or FTP doesn’t necessarily mean a higher durability. (Read more here)

2. Durability’s concept can be applied also to training zones. Specifically, thresholds (and so training zones) change during long rides (power zones move downward and heart rate zones move upward). (Read more here)

To understand how we can improve durability, it’s important to understand first what are its physiological determinants.

Already ~100 years ago and with no data available, the legendary British exercise physiologist, Sir Archibald Hill, noticed that when fatigue during prolonged exercise can be linked to fuel availability. (1)

Carbohydrates are stored in the body mainly as muscle and liver glycogen. While fat is a virtually unlimited resource, unfortunately, glycogen is not. For example (2), a 70kg man with 10% of body fat have stored approximately:

• 2400 kcal as muscle glycogen

• 375 kcal as liver glycogen

• 68000 kcal as fat

Energy stored (kcal) in the body. Calculations are based on Maunder et al. 2018 (2) and consider a 70kg man with a 10% body fat.

While these values could change based on fitness level, genetic and diet, the concept remains the same: we have ~40 times more fats than carbohydrates energy stores.

Depletion of muscle glycogen is linked to fatigue. Specifically, low glycogen levels seems to disrupt the ionic mechanisms that permits muscle contraction (calcium release, sodium and potassium regulation). (3)

Given this context, in 2019 a research group lead by another British exercise physiologist, Prof. Andy Jones (the scientific director of Eliud Kipchoge’s ‘Breaking Two’ historical marathon record sponsored by Nike), tried to answer the following questions (4):

1. Does exogenous carbs ingestion during exercise improve durability?

2. Is muscle glycogen sparing the physiological mechanism behind a superior durability?

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WHAT DID THEY DO?