Endurance Fatigue

"The fat-burning zone is a low-intensity aerobic activity that keeps your heart rate between 60 and 69% of your maximum heart rate. The cardio zone, on the other hand, is a high-intensity aerobic activity that keeps the heart rate between about 70 to 85% of your maximum heart rate. Your body burns a higher percentage of calories from fat during a low-intensity aerobic activity, but there is more to it than that. When you begin a low-intensity activity, about 50% of the calories burned comes from fat, whereas in the cardio zone only 40% comes from fat. However, when looking at the actual numbers of calories burned, higher-intensity activity burns just as much fat and a much greater number of total calories overall.\n\nIf you are familiar with endurance sports, you may have heard of “hitting the wall” or “bonking.” These colloquial terms refer to the extreme fatigue that sets in after about 120 minutes of performing an endurance sport, such as marathon running or long-distance cycling. The physiology underlying “hitting the wall” means that muscles have used up all their stored glycogen and are therefore dependent on other nutrients to support their energy needs. Fatty acids are transported from fat-storing cells to the muscle to rectify the nutrient deficit. However, fatty acids take more time to convert to energy than glucose, thus decreasing performance levels. To avoid “hitting the wall” or “bonking,” endurance athletes load up on carbohydrates for a few days before the event, known as carbohydrate loading. This will maximize an athlete’s amount of glycogen stored in their liver and muscle tissues. It is important not to assume that carbohydrate loading works for everyone. Without accompanied endurance training, you will not increase the amount of stored glucose. If you plan on running a five-mile race for fun with your friend and decide to eat a large amount of carbohydrates in the form of a big spaghetti dinner the night before, the excess carbohydrates will be stored as fat. Therefore, if you are not an endurance athlete exercising for more than 90 minutes, carbohydrate loading will provide no benefit and can even have some disadvantages. Another way for athletes to avoid “hitting the wall” is to consume carbohydrate-containing drinks and foods during an endurance event. Throughout the Tour de France, the average cyclist consumes greater than 60 grams of carbohydrates per hour."