Exercise Physiology: Energy Systems

• Exercise physiology is the study of how the body responds and adapts to physical activity.
• It explores the physiological changes that occur during and after exercise.
• These changes involve multiple systems working together to maintain homeostasis.
• It also includes how these systems adapt over time with regular exercise.

Energy Systems

• Energy systems are biochemical pathways that regenerate adenosine triphosphate (ATP).
• ATP is the primary source of energy for all bodily functions, including muscle contraction.
• The body uses three main energy systems to produce ATP: the phosphagen system, glycolysis, and the oxidative system.
• These systems work together to meet the energy demands of exercise, with their relative contribution varying based on exercise intensity and duration.

Phosphagen System

• The phosphagen system, also known as the ATP-PCr system, provides immediate energy for short-duration, high-intensity activities.
• It relies on the breakdown of creatine phosphate (PCr) to regenerate ATP.
• Creatine kinase is the enzyme that facilitates the reaction: PCr + ADP → ATP + Creatine.
• The phosphagen system can supply energy very rapidly, but its capacity is limited due to the small storage of PCr in muscles.
• It is dominant during activities like sprinting, weightlifting, and jumping.
• This system is active at the start of all exercises, regardless of intensity.
• The phosphagen system is the quickest way to resynthesize ATP.

Glycolysis

• Glycolysis is the breakdown of glucose to produce ATP.
• Glucose can come from blood glucose or muscle glycogen.
• Glycolysis occurs in the cytoplasm and does not require oxygen (anaerobic).
• It involves a series of enzymatic reactions that convert glucose into pyruvate.
• Pyruvate can then be converted into lactate or enter the mitochondria for further oxidation.
• Glycolysis produces a net of 2 ATP molecules when starting from glucose, and 3 ATP molecules when starting from glycogen.
• Glycolysis is important for activities lasting from a few seconds to several minutes, such as middle-distance running or high-intensity interval training.
• The end product of glycolysis, pyruvate, has two possible fates depending on oxygen availability (aerobic vs. anaerobic glycolysis).

Anaerobic Glycolysis

• Anaerobic glycolysis occurs when oxygen supply is limited or when energy demand is high.
• Pyruvate is converted to lactate by the enzyme lactate dehydrogenase.
• This conversion regenerates NAD+, which is needed for glycolysis to continue.
• Lactate accumulation contributes to muscle fatigue and the burning sensation during intense exercise.
• The process allows ATP production to continue at a rapid rate for a short period.
• Lactate can be used as an energy source by other tissues or reconverted to pyruvate in the presence of oxygen
• The Cori cycle involves the transport of lactate to the liver, where it is converted back to glucose through gluconeogenesis.

Aerobic Glycolysis

• Aerobic glycolysis occurs when sufficient oxygen is available.
• Pyruvate enters the mitochondria and is converted to acetyl-CoA.
• Acetyl-CoA enters the Krebs cycle, also known as the citric acid cycle.
• The Krebs cycle produces ATP, NADH, and FADH2.
• NADH and FADH2 then donate electrons to the electron transport chain, where ATP is produced through oxidative phosphorylation.
• Aerobic glycolysis is more efficient than anaerobic glycolysis, producing significantly more ATP per molecule of glucose.

Oxidative System

• The oxidative system is the primary source of ATP at rest and during low-intensity, long-duration activities.
• It occurs in the mitochondria and requires oxygen.
• It involves the Krebs cycle and the electron transport chain.
• The oxidative system can use carbohydrates, fats, and proteins as fuel sources.
• Fat oxidation yields more ATP than carbohydrate oxidation but requires more oxygen.
• This system is dominant during activities like long-distance running, cycling, and swimming.
• Fat becomes the primary fuel source during prolonged, low-intensity exercise.
• The oxidative system is the most efficient way to produce ATP

Fuel Utilization

• During exercise, the body utilizes a mix of carbohydrates, fats, and proteins for energy.
• The relative contribution of each fuel source depends on exercise intensity, duration, and individual factors such as training status and diet.
• Carbohydrates are the primary fuel source during high-intensity exercise.
• Fats are the primary fuel source during low-intensity, long-duration exercise.
• Proteins contribute a small amount to energy production, especially during prolonged exercise or when carbohydrate stores are depleted.
• The crossover concept describes the shift from fat to carbohydrate metabolism as exercise intensity increases.
• Endurance training increases the body's ability to use fat as a fuel source, sparing glycogen and improving performance.
• The respiratory exchange ratio (RER) is used to estimate the proportion of carbohydrate and fat being used as fuel.

Interaction of Energy Systems

• All three energy systems are active during exercise, but their relative contributions vary depending on the intensity and duration of the activity.
• The phosphagen system dominates during short, high-intensity activities.
• Glycolysis becomes more important during activities lasting from a few seconds to several minutes.
• The oxidative system is dominant during long-duration, low-intensity activities.
• As exercise intensity increases, there is a shift from fat to carbohydrate metabolism.
• The energy systems work together to provide a continuous supply of ATP to meet the demands of exercise.
• Energy system training can improve the capacity and efficiency of each system, leading to enhanced athletic performance.
• Interval training involves alternating periods of high-intensity exercise with periods of rest or low-intensity exercise, targeting multiple energy systems.