Exercise Physiology

Understanding energy systems is crucial for designing effective training programs. The body uses three primary systems to produce ATP (adenosine triphosphate), the cellular energy currency.

• ATP-PCr (Phosphagen System): This is the immediate energy system, providing ATP for the first few seconds of high-intensity exercise (e.g., a maximal sprint). It relies on the breakdown of phosphocreatine (PCr) to rapidly regenerate ATP. This system is anaerobic (doesn't require oxygen) and limited by the PCr stores in muscle.

  Example: A 100-meter sprint primarily utilizes the ATP-PCr system.

• Glycolysis: This anaerobic system breaks down glucose (from glycogen in muscles and the liver or from blood glucose) into pyruvate. If oxygen is unavailable (anaerobic glycolysis), pyruvate is converted to lactate. This system provides energy for moderate- to high-intensity activities lasting from 30 seconds to a few minutes.

  Example: A 400-meter run relies heavily on glycolysis.

• Oxidative Phosphorylation (Aerobic System): This is the primary energy system for endurance activities. It uses oxygen to break down glucose, fats, and, to a lesser extent, proteins, to produce ATP. This system is slow but efficient, capable of providing energy for prolonged periods.

  Example: A marathon primarily utilizes the oxidative phosphorylation system.

Interplay: All three systems work together, but their contribution varies depending on intensity and duration. For example, during a 100m sprint, the ATP-PCr system is dominant initially. As the sprint progresses, glycolysis contributes more, while oxidative phosphorylation provides minimal energy. As exercise intensity decreases, the reliance on the oxidative phosphorylation increases.

ATP production rates:

• ATP-PCr System: Very rapid (but limited capacity).
• Glycolysis: Rapid (but produces lactic acid).
• Oxidative Phosphorylation: Slower, but high capacity (requires oxygen).

Exercise intensity is the primary driver of energy system contribution.

• Low Intensity (e.g., walking): Primarily utilizes oxidative phosphorylation (fat oxidation).
• Moderate Intensity (e.g., jogging): A blend of oxidative phosphorylation (glucose and fat) and glycolysis. Oxygen consumption increases steadily.
• High Intensity (e.g., sprinting): Primarily utilizes the ATP-PCr system and glycolysis. Oxygen consumption is insufficient to meet the energy demands (anaerobic metabolism predominates).
• Very High Intensity (e.g., maximal effort): The ATP-PCr system is the dominant provider of ATP at the onset, with glycolysis quickly becoming the primary source, while the aerobic system does provide some contribution.

Anaerobic Threshold/Lactate Threshold: As exercise intensity increases, the body produces more lactate. The anaerobic threshold (also known as lactate threshold) is the point at which lactate production exceeds lactate clearance. Training above this threshold results in a shift toward greater reliance on anaerobic pathways. Determining lactate threshold can be done in the lab using GXT (graded exercise tests).

Hormones play a crucial role in regulating substrate utilization and adaptation during exercise. Understanding these responses is key for optimizing training and nutrition.

• Insulin: Primarily a storage hormone. Exercise, especially at moderate intensity, can increase insulin sensitivity, promoting glucose uptake by muscle cells. Insulin levels decrease during exercise to allow the release of glucose from the liver and fatty acids from adipose tissue.

• Glucagon: Acts in opposition to insulin. It is released during exercise, especially at higher intensities, to stimulate glucose release from the liver (glycogenolysis) and maintain blood glucose levels.

• Cortisol: A stress hormone that increases during exercise, especially in response to prolonged or high-intensity exercise. Cortisol promotes gluconeogenesis (glucose production from non-carbohydrate sources like amino acids) and lipolysis (fat breakdown).

• Catecholamines (Epinephrine & Norepinephrine): Released during exercise. Increase heart rate, blood flow to muscles, lipolysis, and glycogenolysis. They are critical for the 'fight or flight' response and mobilizing energy stores.

• Growth Hormone: Promotes muscle protein synthesis and lipolysis. It typically increases during exercise, particularly with high-intensity or resistance training.

Hormonal interactions: These hormones work in concert to ensure adequate energy supply to meet the demands of exercise, while also promoting adaptations.

Different training modalities elicit distinct metabolic adaptations.

• Endurance Training: Improves oxidative capacity (increased mitochondrial density, capillary density, and aerobic enzyme activity), enhances fat oxidation, and increases glycogen storage capacity. The body becomes more efficient at utilizing oxygen and producing energy aerobically.

• High-Intensity Interval Training (HIIT): Improves both aerobic and anaerobic capacity. It can increase mitochondrial density, improve glycolytic capacity, and enhance lactate tolerance. Promotes greater EPOC (Excess Post-exercise Oxygen Consumption) leading to higher calorie expenditure.

• Resistance Training: Primarily stimulates muscle hypertrophy and strength gains, but also increases glucose uptake, improves insulin sensitivity, and can contribute to increased fat oxidation. Enhances the ATP-PCr system and promotes glycogen storage.

Training Specificity: The adaptations you see with different training modalities are very specific to the modality itself. Training must match the desired performance outcome.