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How Exercise Duration Influences Fuel Selection

During exercise, fuel selection depends on a number of factors, including duration of the exercise. Energy is required for muscle contraction process. During exercise, fatty acids and glucose are the fuels that muscles use during exercise. Their relative contribution and origin is dependent on the duration and intensity of an exercise and other individual traits such as level of physical training. However, worth noting ATP is an energy form that the skeletal muscles ultimately utilize. Triacylglycerol is the form in which fat is stored and CHO is the major storage form for glycogen. Glycogen and triacylglycerol both act as energy reserves. The contribution of CHO and fat during exercise primarily depends on exercise duration and intensity. This essay discusses how exercise duration influences fuel selection.

The oxidation of triacylglycerol and glucose that comes from the skeletal muscle is the main pathway for the re-synthesis of ATP during prolonged periods of exercise. When a light exercise is taking place, the fast-twitch type IIA and slow-twitch type fibers are the ones that are utilized. These fibers are rich in mitochondria, highly oxidative, and highly capillarized both have glycogen and triacylglycerol as substrates. For human subjects, a light workload is defined a workload that demands 30-40% of a person’s VO2 max at a heart rate of 110 beats per minute. The R-value reduces from 0.80 to 0.85 in the initial phase of the exercise to below 0.80 several hours into the exercise. At first, glycogen depletion takes place in type 1 fibres. The magnitude local glycogen stores affect the use and uptake of blood-borne substrates and determine capacity of endurance during high-to-moderate exercise intensity (Evans, Karl, and Brendan, 2857) Glucose uptake from the exercising to blood muscle fibers increases gradually when exercise is prolonged and glucose uptake liver is well matched through release of glucose from the liver.

The importance of glycogen concentration in the liver in a prolonged exercise is depicted by standard-bred trotters performing four h of trotting. At the end of the exercise, liver glycogen had declined from116 mmol/kg to 19 mmol/kg wet weight. Blood glucose gradually declined in the course of exercise, with blood glucose concentration going as low as 1.5 mmol/l. During this exercise period, free fatty acids increased significantly. The low glycogen levels were attained by low blood glucose. During prolonged exercise, that lasts many hours, availability of substrates is important. In a study previously conducted by the FFA, concentration increases and four hours of trotting was three times higher than the others.

Essen-Gustavsson studied substrate utilization and fiber types in skeletal muscles in competing horses over 100 and 50 kilometers. The research found that type 1 fibres used majority of glycogen and 90% of fibres ran out of oxygen following the ride. 78% of type IIA were PAS stained and only 16% type IIB fibres had low stain intensity. It has found that depicted that the best horses performing 50km and 100 km race registered similar triacyglycerol and glycogen in the muscle. This finding shows that horses that go for long races rely to a great extent on extramuscular energy sources, utilizing both lipids and CHO that are derived from both extramuscular and intramuscular sources.

In closing, duration and exercise intensity have unique influences on fuel selection. When low-intensity exercises are taking place, the body uses anaerobic metabolism rather than aerobic metabolism. Exercises that have high intensity tend to burn more calories than low-intensity workouts.

Works Cited

Evans, Mark, Karl E. Cogan, and Brendan Egan. “Metabolism of ketone bodies during exercise and training: physiological basis for exogenous supplementation.” The Journal of physiology 595.9 (2017): 2857-2871.