This page presented by Amazon Exercise & Fitness
Carbohydrate is used as fuel by muscle cells in the form of blood glucose, and has a very limited supply. We keep reserves of Glycogen in muscle and liver tissue that can be tapped for conversion to blood glucose, but it is also a limited supply, so if we are exercising or racing at an intensity that utilizes carbohydrate as the predominant fuel source, we may run out of fuel and "bonk," or "hit the wall." Understanding how your body utilizes fat versus carbohydrate is important in making fueling decisions for training or racing, and may dictate the best maximum aerobic intensity that an athlete can sustain for a given endurance event. An assessment such as a VO2 assessment provides data showing exactly how much fat and carbohydrate are being utilized at any given exercise intensity, and where your first crossover point (where you burn the most fat) and second crossover point (where you no longer burn fat only carbs) is. Once the assessment is completed typically you'll receive training zones with the fat/carb depletion rates. With periodic testing an athlete can monitor their progress and make adjustments to their training and racing strategy.