Energy Systems in Exercise

ATP & PCr System - Energy's First Responders

• ATP (Adenosine Triphosphate): Body's immediate usable energy form.
• PCr (Phosphocreatine): High-energy phosphate; rapidly regenerates ATP.
  • Location: Muscle cytosol.
  • Stores: ~3-5 times more abundant than ATP.
  • Enzyme: Creatine Kinase (CK).
  • Reaction: $PCr + ADP \xleftrightarrow{CK} ATP + Creatine$
  • Duration: Powers maximal efforts for ~5-10 seconds.
• Characteristics:
  • Anaerobic (no $O_2$ required).
  • Fastest ATP production rate; highest power output.
  • Very limited capacity.
  • Dominant in: Sprints (<100m), weightlifting, jumps.
  • Recovery: PCr resynthesis is rapid (aerobic, ~3-5 min for full).

⭐ Depletion of PCr stores is a primary cause of fatigue during all-out exercise bouts lasting 5-10 seconds.

Anaerobic Glycolysis - Sugar Rush Power

• Rapid cytosolic breakdown of glucose or glycogen to pyruvate, no oxygen required.
• Dominant energy system for high-intensity activities lasting 15 seconds to 2 minutes.
• Net ATP Production:
  • From Glucose: 2 ATP (invests 2, yields 4).
  • From Glycogen: 3 ATP (invests 1, yields 4).
• Key rate-limiting enzyme: Phosphofructokinase-1 (PFK-1).
• Under anaerobic conditions, Pyruvate $\xrightarrow{\text{LDH}}$ Lactate. This step regenerates NAD+ for continued glycolysis.
  • Lactic acid $\rightarrow$ Lactate + H+; H+ accumulation contributes to muscle fatigue and acidosis.
• 📌 Mnemonic: "LAG" - Lactate, Anaerobic, Glycolysis.

⭐ Cori Cycle: Lactate from muscle $\rightarrow$ liver for gluconeogenesis $\rightarrow$ glucose back to muscle. Crucial for recovery during and after intense efforts.

Oxidative System - Endurance Engine

• Primary ATP source for endurance activities (>2-3 minutes) & rest; requires oxygen ($O_2$).
• Location: Mitochondria.
• Substrates:
  • Carbohydrates (Glucose/Glycogen) → Pyruvate → Acetyl-CoA.
  • Fats (Triglycerides → Free Fatty Acids) → Acetyl-CoA (via β-oxidation).
  • Proteins (Amino Acids) → Krebs cycle intermediates (minor contribution).
• Key Processes:
  • Aerobic Glycolysis (if starting from glucose).
  • Krebs Cycle (Citric Acid Cycle / TCA Cycle).
  • Electron Transport Chain (ETC) & Oxidative Phosphorylation.
• ATP Yield: High (e.g., ~32 ATP from 1 glucose molecule).
• Rate of ATP Production: Slow.
• Capacity: Very large, virtually limitless (depends on fuel stores).
• Byproducts: Carbon dioxide ($CO_2$), Water ($H_2O$).

⭐ Respiratory Quotient (RQ = $VCO_2 / VO_2$) indicates substrate use: Carbohydrates RQ ≈ 1.0; Fats RQ ≈ 0.7. During prolonged exercise, RQ typically ↓ as fat utilization ↑.

Integrated Metabolism - Fuels & Fatigue Factors

• Fuel Selection (Intensity/Duration Dependent):
  • Low intensity (<30% $VO_2$ max): Fats (FFAs, triglycerides).
  • Moderate (40-60% $VO_2$ max): Mixed fats & CHO (glycogen, glucose).
  • High (>75% $VO_2$ max): CHO (muscle glycogen critical).
    • Crossover concept: CHO use surpasses fat; ↑epinephrine.
  • Prolonged (>90 min): CHO → Fat shift; glycogen sparing.
• Oxygen Dynamics:
  • $O_2$ Deficit: Lag in $O_2$ uptake; anaerobic systems dominate initially.
  • EPOC ($O_2$ Debt):
    • Rapid (Alactacid): Restore ATP, PCr, $O_2$ stores.
    • Slow (Lactacid): Lactate removal (Cori cycle), ↑temp, ↑hormones.
• Fatigue Factors:
  • Peripheral:
    • Metabolites: $H^+$ (↓pH), $P_i$ (↓cross-bridge), $K^+_{ec}$ (↓excitability).
    • Substrates: ↓PCr, ↓glycogen ("hitting the wall").
    • $Ca^{2+}$ handling: Impaired SR release/uptake.
  • Central (CNS): ↓Neural drive; neurotransmitter imbalance.

⭐ The "crossover concept" illustrates that as exercise intensity increases, the body shifts from predominantly fat metabolism to carbohydrate metabolism for ATP production.

High‑Yield Points - ⚡ Biggest Takeaways

• ATP-PCr system: Immediate ATP for ~10 sec maximal efforts (e.g., sprints).
• Anaerobic glycolysis: Dominant for 30s-2min high-intensity exercise; produces lactate.
• Aerobic system: Highest ATP yield from glucose/fats for endurance activities.
• EPOC (Oxygen Debt): ↑ Post-exercise O₂ consumption for recovery.
• Respiratory Quotient (RQ): 1.0 for pure carbohydrate use, ~0.7 for pure fat use.
• Crossover point: Intensity at which carbohydrates become predominant fuel over fats.
• Glycogen depletion: Limits prolonged high-intensity exercise performance.