Energy expenditure must equal energy intake to achieve energy balance. The energy systems used during exercise for muscular work include the phosphagen and glycolytic (both anaerobic) and the oxidative (aerobic) pathways. The phosphagen system is used for events lasting no longer than a few seconds and of high intensity. Adenosine triphosphate (ATP) and creatine phosphate provide the readily available energy present within the muscle. The amount of ATP present in the skeletal muscles (~5 mmol·kg−1 wet weight) is not sufficient to provide a continuous supply of energy, especially at high exercise intensities. Creatine phosphate is an ATP reserve in muscle that can be readily converted to sustain activity for ~3-5 min. The amount of creatine phosphate available in skeletal muscle is approximately four times greater than ATP and, therefore, is the primary fuel used for high-intensity, short-duration activities such as the clean and jerk in weight lifting or the fast break in basketball.
The anaerobic glycolytic pathway uses muscle glycogen and glucose that are rapidly metabolized anaerobically through the glycolytic cascade. This pathway supports events lasting 60-180 s. Approximately 25%-35% of total muscle glycogen stores are used during a single 30-s sprint or resistance exercise bout. Neither the phosphagen nor the glycolytic pathway can sustain the rapid provision of energy to allow muscles to contract at a very high rate for events lasting greater than ~2-3 min.
The oxidative pathway fuels events lasting longer than 2-3 min. The major substrates include muscle and liver glycogen, intramuscular, blood, and adipose tissue triglycerides and negligible amounts of amino acids from muscle, blood, liver, and the gut. Examples of events for which the major fuel pathway is the oxidative pathway include a 1500-m run, marathon, half-marathon, and endurance cycling or ≥1500-m swimming events. As oxygen becomes more available to the working muscle, the body uses more of the aerobic (oxidative) pathways and less of the anaerobic (phosphagen and glycolytic) pathways. Only the aerobic pathway can produce much ATP over time via the Krebs cycle and the electron transport system. The greater dependence on aerobic pathways does not occur abruptly, nor is one pathway ever relied on exclusively. The intensity, duration, frequency, type of activity, sex, and fitness level of the individual, as well as prior nutrient intake and energy stores, determine when the crossover from primarily aerobic to anaerobic pathways occurs.
Conversion of Energy Sources over Time. Approximately 50%-60% of energy during 1-4 h of continuous exercise at 70% of maximal oxygen capacity is derived from carbohydrates and the rest from free fatty acid oxidation. A greater proportion of energy comes from oxidation of free fatty acids, primarily those from muscle triglycerides as the intensity of the exercise decreases. Training does not alter the total amount of energy expended but rather the proportion of energy derived from carbohydrates and fat. As a result of aerobic training, the energy derived from fat increases and from carbohydrates decreases. A trained individual uses a greater percentage of fat than an untrained person does at the same workload. Long-chain fatty aids derived from stored muscle triglycerides are the preferred fuel for aerobic exercise for individuals involved in mild- to moderate-intensity exercise.
Cite this: Nutrition and Athletic Performance - Medscape - Mar 01, 2009.