c. Completely depleting your glycogen stores will result in "hitting the wall." d. Lactate can be used for energy.
There is a drop in the percentage of the energy need that comes from fat oxidation as well as an increase in the percentage that comes from carbohydrate oxidation as relative workout intensity is increased.
As length and intensity rise, carbohydrates (glucose/glycogen) become the main source of energy.Fatty acids will be used as a fuel source if exercise is performed for an extended period of time and glycogen levels are almost depleted.
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