Metabolic changes with exercise

Immediate Energy - Quick Fuel, No Fuss

• Source: Stored ATP and phosphocreatine (PCr) provide instant energy for muscle contraction at the onset of maximal exercise.
• Mechanism: Creatine kinase catalyzes the rapid regeneration of ATP from ADP.
  • $PCr + ADP \leftrightarrow ATP + Creatine$
• Duration: This system sustains all-out effort for only ~10 seconds.
• Oxygen: Anaerobic; does not require oxygen.

⭐ The creatine kinase (CK) system acts as a temporal and spatial buffer for ATP. CK-MB is a marker for myocardial infarction, while CK-MM is the isoform in skeletal muscle.

Short-Term Energy - Sugar Rush, No Air

• Dominant energy system for high-intensity exercise lasting 10 seconds to 2 minutes.
• Primary process: Anaerobic glycolysis, regulated by Phosphofructokinase-1 (PFK-1).
• Fuel: Stored muscle glycogen via glycogenolysis.

• Lactate accumulates, is cleared via the Cori cycle, and contributes to post-exercise oxygen consumption (EPOC), or "oxygen debt."

⭐ Lactate is not merely a waste product; it is a valuable metabolic fuel that can be oxidized by the heart and other muscle fibers or converted back to glucose in the liver.

Long-Term Energy - The Marathon Machine

• Primary system: Powers rest and prolonged exercise lasting > 2 minutes.
• Mechanism: Relies on aerobic metabolism (oxidative phosphorylation) within the mitochondria.
  • Fuel Sources: Glucose (glycolysis), and fatty acids (beta-oxidation) feed into the Krebs cycle and Electron Transport Chain (ETC).
  • Oxygen Supply: Myoglobin functions as an essential O₂ reservoir within muscle tissue.

⭐ The Respiratory Exchange Ratio ($RER = VCO₂/VO₂$) is ~0.7 for pure fat oxidation and 1.0 for pure carbohydrate oxidation. During high-intensity exercise, it can exceed 1.0 due to the buffering of lactic acid.

Hormonal Control - The Hormone Hustle

Fuel Selection - The Crossover Shuffle

• Crossover Concept: Describes the shift from fat to carbohydrate as the primary fuel source as exercise intensity (VO₂ max) increases.
• Low Intensity (<40% VO₂ max): Primarily fats (plasma FFAs, muscle triglycerides).
• High Intensity (>60% VO₂ max): Primarily carbohydrates (muscle glycogen, plasma glucose).
• The crossover point typically occurs at 40-60% VO₂ max, driven by ↑ recruitment of fast-twitch fibers and ↑ epinephrine.

⭐ Endurance training shifts the crossover point to the right, enabling athletes to conserve glycogen by utilizing fats at higher intensities.

High‑Yield Points - ⚡ Biggest Takeaways

• Initial fuel is stored ATP and phosphocreatine, quickly followed by anaerobic glycolysis, then aerobic respiration.
• Key hormones: ↑ glucagon, epinephrine, and cortisol; ↓ insulin to maximize glucose availability.
• Post-exercise oxygen consumption (EPOC) or oxygen debt replenishes ATP/phosphocreatine and metabolizes lactate.
• Cardiac output increases via elevated heart rate and stroke volume to meet oxygen demand.
• Lactate is a fuel source, not just waste; it's used by the heart and liver (Cori cycle).