This is the second part of the talk.
In the first part, we covered some of the basics of energy and energy metabolism,
the different pathways that are available to supply fuel
and to resynthesise ATP during exercise,
and we finished on listing some of the main factors that influence fuel selection for exercise.
In this second talk, I'm going to cover the effects of exercise intensity and
exercise duration on fuel selection for exercise,
and then get into some of the main contributors to the development of fatigue,
or mechanisms of fatigue, that we get during both high-intensity exercise and prolonged exercise.
Then I'll finish with a summary of the main points from both lectures.
We'll now focus on those two main effects of exercise intensity and
duration on the pattern of fuel supply during prolonged exercise.
Fuel selection for exercise.
Fat uptake and oxidation by muscle is a relatively slow process,
so it can only supply ATP at a rate that's sufficient to maintain
exercise at an intensity of about 60% of maximal oxygen uptake
(or aerobic capacity), or less than that.
To generate ATP at the high rates we need to sustain higher exercise intensities than that,
there's an increasing reliance on carbohydrate,
and at intensities above about 85% of VO₂ max (aerobic capacity)
the oxidation of carbohydrate is always going to be
the predominant fuel, contributing at least 70% of the energy (or more), with fat contributing 30% (or less).
The higher you get towards your maximal oxygen uptake (aerobic capacity),
the less and less fat contributes, and the more and more comes from carbohydrate,
that's because both the oxidative pathway of carbohydrate utilisation
and the anaerobic pathway of glycolysis can both supply ATP
at a rate that's much faster than fat oxidation can achieve.