In a recent certification exam, when asked to explain the three energy, two exercises that utilize each pathway, and specific advice related to energy for marathon training, I gave the following response. It is shared here as a resource to be put to use for others. Should questions arise as you review the information, please contact me.
Adenosine Triphosphate (ATP) provides the foundation for energy used in muscle contraction (Hatfield, Fitness: Complete Guide, p. 529). While critical for muscle contraction, a challenge arises in supply as the average individual possesses only 80-100 grams of ATP stored in their entire body, only enough to “perform maximal exercise for a few seconds” (p. 531). Fortunately, pathways exist for the production of additional ATP, offering energy for ongoing muscle contraction.
The first energy pathway is the ATP/PCr System. According to Hatfield, the ATP/Cr pathway is usually the first pathway to kick in (Hatfield, p. 531). This system involves the rapid regeneration of ATP from ADP and P through breaking of the chemical bonds between creatine (Cr) molecules and phosphate molecules already joined in a phosophocreatine molecule (PCr). As the bonds are broken, energy is released freely into the cell and regeneration of ATP is enabled.
As energy demands persist beyond 10 seconds, PCr stores become depleted and the body turns to the glycolytic pathway for the production of energy and regeneration of ATP (Hatfield, p. 532). While relying on the glycolytic pathway, ATP regeneration takes place slowly. The glycolytic pathway is capable of extending energy production for approximately 80 seconds beyond the initial 10 seconds provided by ATP/PCr. The heart of the glycolytic pathway is the body’s extraction of glycogen from existing blood glucose and glycerol found in triglycerides. This process is called glycolysis.
Beyond ATP/PCr and glycolytic pathway production of 90 seconds or so, the body will rely on the oxidative phosphorylative pathway for energy production. According to Hatfield, “as long as oxygen can be utilized, this pathway can continue indefinitely” (p. 532). While the oxidative pathway is the slowest to respond in the production of ATP, “what it lacks in intensity I makes up for in volume” (p. 532), offering sufficient energy for endurance events such as marathons and triathlons.
When training for marathon running, one should take care to know the implications of relying on the oxidative pathway. When operating in the oxidative pathway, a primary concern is the ongoing sources of energy production. While at rest 70% of the body’s energy needs comes from fat, but “as exercise intensity increases, more and more carbohydrates are used instead of fat (Hatfield, p. 23). As intensity continues, the energy coming from carbohydrates nears 100% as beta oxidation of fat cannot keep up with demands (p. 23). When dietary needs are not met, and carbohydrates are not present, the body will “catabolize the very muscle it’s using for energy” (p. 23).
Marathon training calls for careful timing and consumption of high quality carbohydrates to support the needs of the oxidative pathway for long periods of time. In my own personal experience this has provided an important reminder. While studying and preparing for certification, I have been going through a period of cutting weight with a routine of six days cardio added on to a reduced three days of resistance training. In order to retain the muscle size and strength during this phase, it will be important for me to maintain a diet of high quality carbs.
References
Hatfield, Frederick C. Fitness: The Complete Guide. International Sports Sciences Association, Carpinteria, CA. 2016.