Most cells, including muscle cells, contain structures called mitochondria. The mitochondria are the sites of cellular aerobic energy production (also known as cellular respiration, aerobic oxidation and oxidative phosphorylation). Larger numbers of mitochondria in a cell lead to a greater capability for aerobic energy production. Therefore, cells with larger numbers of mitochondria have a greater aerobic capacity and are more resistant to fatigue.
The third pathway for producing ATP involves oxygen. This oxidative phosphorylation (utilizing oxidation and adding a phosphate) or aerobic metabolic pathway utilizes both the Krebs cycle and the electron transport chain.
The Krebs cycle involves a series of chemical reactions to help convert macronutrients (carbohydrates, proteins, and fats) into usable energy (ATP). Oxygen is not involved in the Krebs cycle but it is involved at the end of the electron trans-port chain (a chain of molecules involved in the aerobic production of ATP). The Krebs cycle removes hydrogen molecules from the ingested macronutrients so that these hydrogen molecules can be used to complete the electron transport chain along with oxygen, an inorganic phosphate, and ADP to form ATP. This process will form a net gain of two more ATP molecules to be used for energy (the process also involves two carrier molecules known as NAD and FAD which transport the hydrogen molecules). During the activation of the electron transport chain in the aerobic production of ATP free radicals are formed.
Free radicals are molecules that now have an unpaired electron as a result of the oxidative phosphorylation of or aerobic production of ATP. Free radicals are highly volatile and reactive and will bind quickly to other molecules and damage the cells they contact. The number of free radicals produced is directly linked to the rate of aerobic metabolism (which is dependent upon the conditioning level of the individual combined with the current exercise intensity and duration).
The breakdown of fat to yield ATP is known as lipolysis. Fat is the most abundant energy source available to the muscle, and the predominant energy source used during the aerobic oxidation phase of energy production. There is a virtually unlimited supply of fatty acids, but the rate at which the body can metabolize them is the limiting factor in obtaining ATP.
Lipolysis is the primary source for energy production during rest and low level activity. But, it’s input to the overall muscular energy sup-ply will decrease as the intensity of muscular contraction in-creases. When lipolysis is unable to meet the increasing energy requirement of exercise (it is too slow to supply enough energy), glycogen is used from the liver and muscles. Once glycogen depletion occurs, exercise intensity will decline.
All exercise intensities (including rest or no activity) that can be maintained for 2 minutes or longer utilizes the oxidative system for energy production. Once approximately 10 minutes of continuous exercise has elapsed, glycolysis is no longer significantly involved in energy production and the oxidative energy system supplies almost all cellular energy for the muscles.