Energy systems during exercise

Energy Systems - The Body's Power Plants

• Universal Energy Currency: Adenosine Triphosphate ($ATP$) powers all muscle contraction. The body regenerates $ATP$ via three primary systems based on exercise intensity and duration.

• System Timeline:

  • Phosphagen (Immediate): Creatine phosphate fuels maximal efforts for <10 seconds.
  • Glycolytic (Short-term): Anaerobic glycolysis sustains high-intensity work for 1-2 minutes.
  • Oxidative (Long-term): Aerobic metabolism supports endurance activities.

⭐ The "crossover concept" illustrates the point at which fat, instead of carbohydrate, becomes the predominant fuel source during prolonged, lower-intensity exercise.

Phosphagen System - The Immediate Power Burst

• Primary Fuel: Stored ATP & Phosphocreatine (PCr), providing the most rapid ATP regeneration.
• Activity Type: All-out, explosive efforts like 100m sprints, shot put, or heavy weightlifting.
• Duration: The dominant energy source for the initial ~10-15 seconds of maximal exercise.
• Key Reaction: The creatine kinase enzyme catalyzes the breakdown of PCr to resynthesize ATP.
  • $PCr + ADP \xrightarrow{Creatine Kinase} ATP + Cr$
• 📌 Mnemonic: Powerful Creatine Reaction for immediate power.

⭐ This system is anaerobic and alactic. It operates without oxygen and crucially, does not produce lactate, allowing for rapid recovery before the glycolytic system dominates.

Glycolysis - The Anaerobic Bridge

• Dominant energy system for high-intensity activities lasting 15s to 2 min (e.g., 400m sprint).
• Anaerobic breakdown of glucose in the cytoplasm.
  • Net Yield: $Glucose \rightarrow 2 Pyruvate + 2 ATP + 2 NADH$.

• Pyruvate's Fate:
  • Anaerobic: Converts to lactate, regenerating NAD+ for continued glycolysis.
  • Aerobic: Enters mitochondria to become Acetyl-CoA for the Krebs cycle.

⭐ Cori Cycle: Lactate from muscles travels to the liver, is converted back to glucose (gluconeogenesis), and returns to muscles for fuel.

Oxidative Phosphorylation - The Endurance Engine

• Primary energy system for long-duration, low-to-moderate intensity activities (>2 minutes).
• Location: Mitochondria.
• Fuel: Primarily fatty acids and glucose.
• Process: Aerobic; requires oxygen ($O_2$) as the final electron acceptor.
• Yield: Extremely high ATP output (~32-38 ATP/glucose), but the slowest system to respond.

⭐ Crossover Concept: As exercise duration increases and intensity remains low-to-moderate, the body shifts from carbohydrate to fat as the primary fuel source to conserve limited glycogen stores.

Energy Continuum - The Systems Hand-Off

• Energy delivery is a dynamic overlap of all three systems, not a sequential hand-off.
• The contribution of each system is dictated by exercise intensity and duration.
  • ATP-PCr System: Dominant for initial, high-power bursts (<10 sec).
  • Glycolytic System: Bridges the gap, peaking at ~45-90 sec.
  • Oxidative System: Primary source for sustained, lower-intensity efforts (>2 min).

⭐ The "crossover concept" illustrates the shift from fat to carbohydrate as the predominant fuel source as exercise intensity increases.

High‑Yield Points - ⚡ Biggest Takeaways

• ATP-PCr system provides instant energy for short, explosive bursts (<10 sec) like sprints.
• Anaerobic glycolysis dominates for activities lasting 10-90 seconds, producing lactate and causing fatigue.
• Aerobic respiration is the primary source for prolonged, endurance exercise (>2 min).
• The crossover concept is the shift from fat to carbohydrate metabolism as exercise intensity increases.
• RER of 1.0 indicates pure carbohydrate use; RER of 0.7 indicates pure fat use.
• EPOC repays the oxygen debt to replenish ATP/PCr and clear lactate.