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Wednesday, August 9, 2017

Metabolic energy pathways provide the energy for your workouts



















The human body utilizes three metabolic energy systems to replenish adenosine triphosphate (ATP). ATP provides energy for all movements, exercises, and muscular activity. These energy systems include the phosphagen system, the glycolysis system, and the oxidative system. The phosphagen system, and the glycolysis system are anaerobic, and do not require oxygen. The glycolysis system is subdivided into two sub systems, which are fast glycolysis and slow glycolysis. The oxidative system is aerobic and requires oxygen. The three macronutrients, proteins, fats, and carbohydrates, are the main food sources for energy. Carbohydrates are the only macronutrient that can be utilized for energy, without oxygen. All physical movements and exercises require the use of one or more of the three energy systems. The type and duration of the movement, or exercise determines which energy system will be utilized (Coburn & Malek, 2012).

For short duration, high intensity, explosive, power movements, the phosphagen system is utilized. The phosphagen system is used for about the first ten seconds of activity. Examples of activities that rely on the phosphagen system include short sprints, plyometric actions such as box jumps, and powerful actions such as an Olympic lift. Even if the physical activity lasts longer than ten seconds, the phosphagen system acts first. ATP and creatine phosphate are the primary elements used by the phosphagen system. The enzymes myosin adenosine triphosphatase (ATPase) and creatine kinase play a role in the breakdown, and regeneration of ATP. The ATP is broken down to release its energy. Myosin ATPase facilitates the breakdown of the ATP into adenosine diphosphate (ADP) and phosphate. This catabolic action releases the energy from the ATP so that it can be utilized by the body. Once this occurs, the ATP must be regenerated. The adenosine diphosphate (ADP) levels rise, which activates the creatine kinase. The creatine kinase breaks down the creatine phosphate, which provides a phosphate group to combine with the ADP. This anabolic sequence produces ATP. This cycle repeats rapidly, and provides a high rate of energy, for a very short duration (Karp, 2009).

As physical exertion moves beyond the first ten seconds, the glycolysis system activates. Glycolysis breaks down carbohydrates from blood glucose, and stored glycogen to form ATP. The glycolysis system provides energy for high intensity activity, which lasts up to two minutes. For intense activities lasting for about thirty seconds, fast glycolysis is used. With fast glycolysis, pyruvate is converted to lactate, to produce ATP. The lactate can be then sent to the liver to be converted back into glucose through the Cori cycle. As the activity goes beyond thirty seconds, slow glycolysis begins to take over. With slow glycolysis, the pyruvate is sent to the mitochondria, through the Krebs cycle, to produce more ATP. This occurs from approximately thirty seconds to two minutes of intense exercise. The intensity of the activity or exercise will decline as the sequence moves through the phosphagen system, to fast glycolysis, and on to slow glycolysis. Examples of activities that could utilize the glycolysis system include a four hundred yard run, a high repetition set of a barbell exercise, or a full court press in basketball (Kelso, 2017).

At approximately the two minute mark and beyond, the oxidative system takes over from the slow glycolysis system. This is where the activity switches from anaerobic to aerobic, and oxygen is required. The oxidative system utilizes both carbohydrates and fats to fuel ATP production. Protein is not usually metabolized as fuel, unless the activity lasts for over ninety minutes. With the oxidative system, ATP production can occur through the Krebs cycle, electron transport chain, and beta oxidation. In addition to being used to produce ATP for longer, slower activities, the oxidative system also works while at rest. While at rest, fats are predominantly used as fuel. Once activity begins, it switches to carbohydrates. Once glucose levels start to become depleted, the system reverts back to fats. As the glycolysis system fades and the oxidative system takes over, the pyruvate is sent to the mitochondria, and converted into acetylCoA. The acetylCoA is then sent through the Krebs cycle for more ATP production. As glucose levels decline, fats are metabolized through the electron transport chain, and beta oxidation process, to be converted to acetylCoA. The acetylCoA then enters the Krebs cycle to produce more ATP. If the activity is very long in duration, protein can assist in energy production through gluconeogenesis and the Krebs cycle. The protein is broken down into its amino acids, which are then either converted to glucose or acetylCoA. Protein breakdown is usually minimal, as glucose and fats are normally present in sufficient quantities. for most activities. Examples of activities utilizing the oxidative system include rest, medium to long distance runs, triathlons, and manual labor during a work day (Pegg, 2013).

References:

Coburn, J.W., & Malek, M.H. (2012). NSCA’s essentials of personal training (2nd ed.). Champaign, IL: Human Kinetics.

Karp, J. (2009). The three metabolic energy systems. Idea Fit. Retrieved from http://www.ideafit.com/fitness-library/the-three-metabolic-energy-systems

Kelso, T. (2017). Understanding energy systems. Breaking Muscle. Retrieved from https://breakingmuscle.com/fitness/understanding-energy-systems-atp-pc-glycolytic-and-oxidative-oh-my

Pegg, A. (2013). What is the oxidative energy system? Steady Strength. Retrieved from http://steadystrength.com/glossary/oxidative-energy-system/

Eric Dempsey
MS, NASM Fitness Nutrition Specialist
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