Muscle Metabolism During Exercise

ATP & PCr System - Instant Energy Burst

• ATP (Adenosine Triphosphate):
  • Immediate energy currency.
  • Muscle stores fuel ~2-3 sec of maximal activity.
  • Reaction: $ATP \rightarrow ADP + P_i + Energy$.
• PCr (Phosphocreatine):
  • Rapidly regenerates ATP. 📌 Mnemonic: PCr = "Provides Cash rapidly" (ATP as cash).
  • Reaction: $PCr + ADP \stackrel{\text{Creatine Kinase}}{\longleftrightarrow} ATP + Creatine$.
  • Extends maximal effort to ~10 sec total.
• System Features:
  • Anaerobic (alactic).
  • Highest power output.
  • Limited capacity (depletes quickly).
  • Fuels 100m sprint, weightlifting.

⭐ The ATP-PCr system provides energy for maximal intensity exercise lasting about 5-10 seconds, e.g., a 100m sprint or heavy weight lift.

Glycolysis - Sugar Breakdown Sprint

• Rapid anaerobic ATP production in cytosol; fuels high-intensity efforts (~15s to 2 min).
• Substrates: Glucose (from blood) or Muscle Glycogen (faster mobilization).
• Net ATP Yield:
  • Glucose: 2 ATP
  • Glycogen: 3 ATP (bypasses 1 ATP-use step)
• Key Products: 2 Pyruvate, 2 $NADH$ per glucose.
• Rate-limiting enzyme: Phosphofructokinase-1 (PFK-1).
• Pyruvate's Fate (depends on O₂ availability & intensity):
  • Anaerobic (O₂ limited / high demand): Pyruvate → Lactate (via LDH).
    • Regenerates $NAD^+$ to sustain glycolysis.
    • Lactate can be oxidized or enter Cori cycle.
  • Aerobic (O₂ sufficient): Pyruvate → Acetyl-CoA → enters Krebs Cycle.

⭐ Lactate threshold (LT) or Onset of Blood Lactate Accumulation (OBLA at 4 mmol/L) is a key indicator of endurance performance and can be significantly improved with training.

Oxidative Phosphorylation - Marathon Fuel Factory

• Primary ATP source for endurance activities (e.g., marathons, long-distance cycling).
• Location: Mitochondria. Requires O₂ (aerobic).
• Fuels: Primarily pyruvate (from glucose) and fatty acids; amino acids contribute minimally.
• Key Stages:
  • Pyruvate converted to Acetyl-CoA.
  • Acetyl-CoA enters Krebs Cycle (Citric Acid Cycle): generates ATP, NADH, FADH₂.
  • Electron Transport Chain (ETC):
    • NADH and FADH₂ donate electrons.
    • Energy release creates a proton gradient across inner mitochondrial membrane.
    • ATP Synthase uses this gradient to produce ATP (Chemiosmosis).
• Net ATP Yield:

  ⭐ Complete aerobic oxidation of one glucose molecule (glycolysis, pyruvate decarboxylation, Krebs cycle, ETC) yields approximately 32 ATP molecules.

• Slower ATP production rate vs. anaerobic systems, but vast, sustainable capacity if fuel and O₂ are available.
• Endurance training ↑ mitochondrial density and enzyme activity, enhancing oxidative capacity.

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Fuel Selection & Hormones - Metabolic Traffic Control

• Fuel Choice Factors:
  • Intensity & Duration: Low intensity/long duration → Fats; High intensity/short → CHO.
  • Training Status: Trained use fats more efficiently.
  • Diet: Impacts glycogen stores.
• Respiratory Exchange Ratio (RER): $VCO_2 / VO_2$
  • ~0.7: Predominantly Fat oxidation.
  • ~1.0: Predominantly CHO oxidation.

⭐ The 'crossover concept' describes the shift from fat to carbohydrate metabolism as exercise intensity increases; Respiratory Exchange Ratio (RER) approaches 1.0 with higher intensity (CHO use) and 0.7 at rest/low intensity (fat use).

• Key Hormonal Shifts:
  • Insulin: ↓ (↑ glucose & FFA release).
  • Glucagon: ↑ (↑ liver glucose production).
  • Catecholamines (Epi, NE): ↑↑ (↑ glycogenolysis, ↑ lipolysis).
  • Cortisol: ↑ (long/intense exercise; ↑ protein breakdown, ↑ lipolysis).
  • Growth Hormone (GH): ↑ (↑ FFA mobilization, ↑ gluconeogenesis).

High-Yield Points - ⚡ Biggest Takeaways

• ATP is the immediate energy currency; creatine phosphate buffers ATP for the initial ~10 seconds of intense activity.
• Anaerobic glycolysis (glucose → lactate) provides rapid ATP for short bursts (~1-2 minutes), producing lactate and contributing to muscle fatigue (H+).
• Aerobic metabolism (oxidation of glucose and fatty acids) is the primary ATP source during prolonged, submaximal exercise.
• Fuel utilization shifts: Carbohydrates → Fats with ↑duration & ↓intensity of exercise.
• EPOC (Excess Post-exercise Oxygen Consumption) or oxygen debt is crucial for restoring ATP/CP, metabolizing lactate, and replenishing O2 stores.
• Type I (slow-twitch) fibers are highly oxidative and fatigue-resistant; Type IIx (fast-twitch) fibers are primarily glycolytic, powerful, but fatigue quickly.