Fundamentals of Exercise Physiology
Introduction
Just as a a car requires fuel to run and machines need electricity to function, all movement and physical work require energy. The human body is no exception. As a biologically powered machine operating through electrical signals, the body continuously requires energy, which must be supplied through metabolic processes.
To meet this demand, the body relies on adenosine triphosphate (ATP) as its primary energy currency. ATP provides immediate energy for cellular activities and is generated through three main metabolic pathways.
Major Pathways of Energy Metabolism
The human body produces energy through three primary metabolic systems. Each pathway differs in terms of energy source utilization, ATP production speed, metabolic location, and duration of energy supply.
- ATP-PCr System (Phosphagen System)
- Glycolysis
- Aerobic Metabolism in the Mitochondria (TCA Cycle, Oxidative Phosphorylation, etc.)
(1) ATP-PCr System (Phosphagen System)
The ATP-PCr system is the body's immediate energy source, activated at the onset of high-intensity exercise. This pathway utilizes phosphocreatine (PCr) stored in muscles. When PCr is broken down into creatine (Cr) and an inorganic phosphate (Pi), the released energy is used to regenerate ATP. Since this process does not require oxygen, it is classified as an anaerobic pathway and is the fastest way to produce ATP. However, due to the limited storage of PCr in muscles, its duration is very short, making it suitable for activities like sprinting or powerlifting that last fewer than 10 seconds.
Key Characteristics:
- Fastest ATP production method
- Ideal for short bursts of high-intensity activity (e.g., sprinting, powerlifting)
Mechanism:
- Utilizes stored phosphocreatine (PCr) in muscles
- PCr breakdown releases energy to convert ADP into ATP
- Anaerobic metabolic process
Duration:
- Very short due to limited PCr availability
- Predominantly active in high-intensity efforts lasting less than 10 seconds
(2) Glycolysis
Glycolysis is a metabolic pathway that utilizes glucose from glycogen stores in muscles and the bloodstream to generate ATP. This process occurs in the cytoplasm of cells and can proceed either aerobically or anaerobically, depending on oxygen availability.
During glycolysis, glucose is broken down into two molecules of pyruvate, releasing energy in the process. While glycolysis produces ATP at a relatively fast rate, it is less efficient compared to aerobic metabolism. Each molecule of glucose yields only 2–3 ATP molecules.
If sufficient oxygen is available, pyruvate enters the mitochondria for further oxidation, producing additional ATP. However, in oxygen-limited conditions, pyruvate is converted into lactate. Glycolysis is the predominant energy system for activities lasting between 20 seconds and 2 minutes, such as middle-distance running and weightlifting.
Key Characteristics:
- Occurs in the cytoplasm
- Can function aerobically or anaerobically
- Faster than aerobic metabolism but less efficient
- Best suited for moderate-intensity activities
Mechanism:
- Glucose (from glycogen or blood sugar) is broken down into pyruvate
- Yields 2–3 ATP per glucose molecule
- If oxygen is insufficient, pyruvate is converted to lactate
- If oxygen is available, pyruvate enters the mitochondria for further energy production
Duration:
- Primarily active in exercises lasting 30 seconds to 2 minutes
- Can contribute to energy supply for longer durations depending on intensity and oxygen availability
(3) Aerobic Metabolism in the Mitochondria (Oxidative Phosphorylation)
While he ATP-PCr system and glycolysis generate ATP quickly, they are relatively inefficient for sustained energy demands. To support prolonged activity, the body relies on aerobic metabolism, which takes place in the mitochondria—often referred to as the body's "powerhouses."
Mitochondria utilize pyruvate from glycolysis, along with fatty acids and amino acids, to produce ATP. This process is highly efficient: each pyruvate molecule generates approximately 15 ATP, while fatty acids can yield over 100 ATP per molecule. Since aerobic metabolism allows for the use of fat stores, it can theoretically provide an unlimited energy supply for extended activities. However, it requires a stable oxygen supply and takes longer to initiate compared to anaerobic pathways. Consequently, it is the primary energy system for prolonged, low- to moderate-intensity exercise, such as marathon running.
Key Characteristics:
- Occurs in mitochondria
- Slowest but most efficient ATP production method
- Suitable for prolonged, low-intensity exercise
- Requires oxygen and can utilize various energy substrates (glucose, fats, amino acids)
Mechanism:
- Oxidizes substrates into water and carbon dioxide, releasing energy to produce ATP
- Produces approximately 28–30 ATP per glucose molecule (15 ATP per pyruvate)
- Fat oxidation generates over 100 ATP per fatty acid molecule
Duration:
- Predominantly active in exercises lasting over 2 minutes
- Best suited for low-intensity, long-duration activities
- Utilizes fat stores, making energy supply virtually limitless
- Effectiveness depends on oxygen availability and exercise intensity
Interaction Between Energy Systems
Although running is often considered an aerobic activity, mitochondrial metabolism is not the sole energy pathway at play. When running begins, the ATP-PCr system is the first to be activated, followed by glycolysis, and then mitochondrial metabolism. Even as mitochondrial metabolism takes over, glycolysis continues to function concurrently to support energy demands.
As exercise intensity increases, glycolysis becomes the dominant pathway, yet aerobic metabolism continues to contribute through oxygen uptake. Additionally, creatine phosphate can still be utilized to replenish ATP. This demonstrates that no single pathway operates in isolation; rather, they interact dynamically based on exercise intensity and duration.
Because the primary ATP production pathway varies depending on exercise duration and intensity, it is challenging to define a single dominant system for all activities. The three energy systems work in an interconnected and complementary manner. The human body has evolved to optimize energy production based on immediate needs, ensuring efficient energy supply across various conditions.
To enhance exercise performance, it is essential to understand the current intensity and environment of physical activity, the primary metabolic pathways in use, the mechanisms behind energy production, and the appropriate nutrients required to support these metabolic processes.
References
Books
1) Amon, Harvey Lodish, Lodish, Berk, Kaiser, Krieger, Bretscher, Ploegh, Martin, Yaffe, Michael B., 고재원, 신동혁, 양원호, 이선경, 이승택, 이주헌, 이태호, 이한웅, ... Yaffe, Michael B. (2023). Molecular Cell Biology / Authors: Harvey Lodish [et al.]; Chief Translator: Jo Jin-won (9th ed.). World Science.
2) Berg, Tymoczko, Stryer, 박인원, Tymoczko, John L., & Stryer, Lubert. (2012). Biochemistry / Authors: Jeremy M. Berg, John L. Tymoczko, Lubert Stryer; Chief Translator: Park In-won (7th ed.). Bumpoon Education.
3) Brooks, Fahey, Baldwin, 정영수, 김현주, 김연수, 박헌, 서상훈, 송욱, 이동규, 전용관, 전태원, Fahey, Thomas D., & Baldwin, Kenneth M. (2010). Exercise Physiology / Authors: George A. Brooks, Thomas D. Fahey, Kenneth M. Baldwin; Translators: Jeong Young-soo [et al.] (4th ed.). Rainbow Books.
4) Hargreaves, M., & Spriet, L. L. (2006). Exercise Metabolism. Human Kinetics.
5) Kenney, W. L., Wilmore, J., & Costill, D. L. (2019). Physiology of Sport and Exercise (7th ed.). Human Kinetics.
6) McArdle, W. D., Katch, F. I., & Katch, V. L. (2014). Exercise Physiology: Nutrition, Energy, and Human Performance (8th ed.). Lippincott Williams & Wilkins.
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Articles
1) Hargreaves, M., & Spriet, L. L. (2018). Exercise metabolism: Fuels for the fire. Cold Spring Harbor Perspectives in Medicine, 8(8), a029744.
2) Hargreaves, M., & Spriet, L. L. (2020). Skeletal muscle energy metabolism during exercise. Nature Metabolism, 2(9), 817–828.