There are three metabolic energy pathways used by the body to produce energy to support physical activity. Each energy pathway stores, utilizes, and resynthesizes ATP in different ways. The three energy pathways are the phosphagen, glycolysis, and oxidative systems. The phosphagen, and glycolysis systems are anaerobic, and do not use oxygen. The oxidative system is aerobic, and does use oxygen. Each energy pathway is specifically used for a certain type of activity, and for a certain duration of time. All three energy pathways work in concert throughout physical events to ensure the body’s energy demands are met. Normally, no one single pathway provides all of the energy requirements during physical activity (Haff & Triplett, 2016).
The phosphagen system process occurs in the sarcoplasm. It is utilized by activities that require a maximal output of strength and power, for a very short duration, typically in the 0-6 second range. Powerlifting, and Olympic weightlifting are two examples of sports, which utilize the phosphagen system Adenosine triphosphate (ATP) is the body’s main source of energy. It is utilized for basic cellular functions, as well as for all movement, and muscular actions. The body cannot store large amounts of ATP. There is approximately 80-100 grams of ATP stored at any given time (Haff & Triplett, 2016).
With the phosphagen system, ATP has to be resynthesized rapidly, in order to meet the body’s energy demands. When ATP is utilized, it is broken back down into adenosine diphosphate (ADP) and creatine phosphate (CP). The ADP and CP are then synthesized back into ATP, with the help of the enzyme, creatine kinase. This creatine kinase reaction quickly replenishes the ATP stores. The phosphagen system has the fastest rate of ATP production, and the least capacity for ATP production, of the three energy pathways (Haff & Triplett, 2016).
When the physical activity moves into the 6-30 seconds window, a combination of both the phosphagen and glycolysis systems are utilized. This combination is used for high intensity activities requiring high energy output. An example of a sport utilizing this combination would be track and field, with a variety of short distance sprinting, relays, and hurdles. At approximately the 30 second mark, the glycolysis system takes over until the 2 minute mark. The glycolysis system process also occurs in the sarcoplasm (Franchini, Takito, & Kiss, 2016).
The glycolysis system is slower than the phosphagen system, but has a much larger capacity for ATP production. It involves the breakdown of carbohydrates from muscle glycogen, and glucose found in the blood. This breakdown of carbohydrates is ultimately used to resynthesize ATP. Glycolysis using blood glucose produces two ATP molecules. The glycolysis process using muscle glycogen produces three ATP molecules. The glycolysis system is complex, and uses two main routing methods or pathways (Franchini et al., 2016).
These are referred to as fast or anaerobic, and slow or aerobic glycolysis. The end result of glycolysis is the production of pyruvate. The fast glycolysis pathway changes pyruvate into lactate, in the sarcoplasm. This causes the ATP resynthesis to happen much faster, but for a shorter period of time. A secondary lesser used route, for faster ATP resynthesis, uses the Cori cycle. Lactate can be moved through the blood to the liver, and then it is changed into glucose. The Cori cycle helps to get more glucose back into the glycolysis process, when blood lactate levels are rising (Haff & Triplett, 2016).
Slow or aerobic glycolysis involves transporting the pyruvate into the mitochondria. The pyruvate enters the Krebs cycle, in order to resynthesize ATP. When this route is utilized, the ATP resynthesis rate takes longer, but it can go on for a longer period of time. Which process is used by the body depends on the energy requirements at that time. When ATP is needed quickly, the fast glycolysis is used. When the exercise intensity is a little lower, the slow glycolysis can be used for a longer period of time (Sousa, Vasque, & Gobatto, 2017).
During the 2-3 minutes window, during activities of a moderate intensity, a combination of the glycolytic and oxidative systems is utilized to produce ATP. Short distance running would be an example of an activity, which would utilize this combination (Sousa et al., 2017).
Once the activity passes 3 minutes, the oxidative or aerobic system takes over. The oxidative system then becomes the primary source of ATP production. The oxidative system is primarily utilized during periods of rest, and during low intensity activities, of longer duration. It utilizes fats and carbohydrates as its main substrates. Protein is not a major contributor to total energy. But it is used in greater quantities during aerobic events which last 90 minutes or longer. The oxidative system breaks down the three macronutrients, or substrates, into acetyl-CoA. The acetyl-CoA then enters the Krebs cycle to produce ATP. The oxidative system, starts with glycolysis, and also uses the Krebs cycle, and the electron transport chain (Sousa et al., 2017).
The breakdown of one molecule of blood glucose results in the production of approximately 38 ATP molecules. If the glycolysis uses muscle glycogen, the count rises to 39 ATP molecules. Fat provides large amounts of ATP. The breakdown of a single triglyceride molecule can produce over 300 ATP molecules. This provides more energy for the long, steady state aerobic events. Marathons and triathlons are examples of sports, which utilize the oxidative system (Sousa et al., 2017).
To maximize athletic performance, training should be based around the sport requirements and the supporting energy pathway. Some athletes need to spend time training in all 3 energy pathways, while others should focus only on the anaerobic pathways. For example, combat sport athletes need to train in each energy pathway singularly, and progressively. These athletes spend different amounts of time in each energy pathway window, and collectively move through all three during a competitive bout. Specific training that is tied to energy pathways, will allow the athlete to perform at optimal levels throughout their competition event (Martins et al., 2018).
References:
Franchini, E., Takito, M., & Kiss, M. (2016). Performance and energy systems contributions during upper-body sprint interval exercise. Journal of Exercise Rehabilitation. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5227314/
Haff, G., & Triplett, N. (Eds.). (2016). Essentials of strength training and conditioning (4th ed.). Champaign, IL: Human Kinetics.
Martins, E., Ricardo, J., De-Souza-Ferreira, E., Camacho-Pereira, J., Ramos-Filho, D., & Galina, A. (2018). Rapid regulation of substrate use for oxidative phosphorylation during a single session of high intensity interval or aerobic exercises in different rat skeletal muscles. Comparative Biochemistry and Physiology. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/29222029
Sousa, F., Vasque, R., & Gobatto, C. (2017). Anaerobic metabolism during short all-out efforts in tethered running: Comparison of energy expenditure and mechanical parameters between different sprint durations for testing. Plos One. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5466345/
Eric Dempsey
MS, ISSA Master Trainer
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