Energy expenditure must be equal to 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 for no longer than a few seconds and high intensity. Adenosine triphosphate, ATP, and Creatine phosphate provide the readily available energy present within the muscle. The amount of ATP present in skeletal muscles is not sufficient to provide a continuous reserve in muscle that can be readily converted to sustain activity for 3-5 minutes. The amount of Creatine phosphate available in skeletal muscle is 4 times greater than ATP and therefore the primary fuel for high intensity, short duration activities such as the clean and jerk in weightlifting or 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 <120 seconds. Approximately 25-35 percent of total muscle glycogen stores are used during a single 30-second sprint or resistance exercise bout. Neither the phosphagen nor the glycolytic pathway can sustain the rapid provision of the energy to allow muscles to contract at very high for events lasting greater than 2 to 3 minutes.
The oxidative pathways fuels events lasting longer than 2 to 3 minutes. The measure 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 pathways is the oxidative pathway include a 1500-meter run, marathon, half-marathon, and endurance cycling or >=500 meter 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 pathways can produce large amounts of ATP over a time viz the Kerb’s cycle and the electron transport system.
The greater energy dependence upon 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 Overtime
Approximately 50 to 60 percent of energy during 1 to 4 hours of continuous exercise at 70 percent of maximal oxygen capacity is derived from carbohydrates and the rest from free fatty acid oxidation. A greater proportion of energy comes from the oxidation of free fatty acids, primarily those from muscle triglyceride 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 increase and carbohydrates decreases.
A trained individual uses a greater percentage of fat than an untrained person does at the same workload. Long chain fatty acids derived from stored muscle triglycerides are the preferred fuel for aerobic exercise for individuals involved in mild to moderate intensity exercise.