Muscle Metabolism and Fatigue

Muscle Energy Systems - Fueling the Flex

Muscles use four primary systems for ATP, vital for varying exercise intensities and durations:

• ATP Stores:
  • Immediate, direct energy source.
  • Lasts only ~2-3 seconds.
• Phosphocreatine (PCr) System:
  • Rapidly regenerates ATP: $ADP + PCr \leftrightarrow ATP + Cr$.
  • Powers maximal efforts for ~8-10 seconds.
• Anaerobic Glycolysis:
  • Glucose breakdown without $O_2 \rightarrow$ Lactic Acid + 2 ATP.
  • Dominant for high-intensity activities lasting ~30 seconds to 2 minutes.
• Aerobic (Oxidative) System:
  • $O_2$ dependent; in mitochondria.
  • Fuels: Glucose, fatty acids, amino acids.
  • Primary source for endurance activities (>2 minutes); yields ~32 ATP/glucose.
  • 📌 Recruitment order: ATP stores $\rightarrow$ PCr system $\rightarrow$ Anaerobic glycolysis $\rightarrow$ Aerobic system.

⭐ Fatty acids are the main fuel for the aerobic system during prolonged, low-intensity exercise (e.g., marathons).

EPOC & Recovery - Huff & Puff Payback

Excess Post-exercise Oxygen Consumption (EPOC), or "oxygen debt," repays the body's oxygen deficit after strenuous activity. It has two main phases:

• Alactacid (Rapid) Component:
  • ATP & Phosphocreatine (PCr) restoration.
  • Myoglobin oxygenation.
  • Constitutes about 20% of total EPOC.
• Lactacid (Slow) Component:
  • Lactic acid removal (conversion to glucose/pyruvate, oxidation).
  • Glycogen resynthesis.
  • Elevated metabolic rate due to ↑ temperature & hormones.
  • Forms about 80% of total EPOC.

⭐ During the lactacid phase of EPOC, a significant portion of lactic acid is converted back to glucose in the liver via the Cori cycle, consuming ATP in the process.

Fiber Type Metabolism - Metabolic Personalities

Muscle fibers are metabolically specialized, impacting contraction speed, power, and endurance.

• Fibers are recruited based on Henneman's Size Principle: Type I → Type IIa → Type IIx/IIb, from smallest to largest motor units.

⭐ Exam Favourite: Athletes' fiber type composition adapts to training; endurance training ↑ Type I characteristics, while sprint training ↑ Type II characteristics.

oka

Mechanisms of Fatigue - The Wall Hits Hard

Fatigue: ↓ force/power output. Sites: Central (CNS drive ↓) & Peripheral (muscle level).

Peripheral Fatigue Mechanisms:

• Metabolite Buildup:
  • $H^+$ (Lactic acid): ↓ pH, inhibits enzymes (e.g., PFK), ↓ Ca²⁺ sensitivity.
  • $P_i$ (Inorganic phosphate): ↓ SR Ca²⁺ release & sensitivity, ↓ cross-bridge force.
• Fuel Depletion:
  • Glycogen stores ↓: Limits ATP regeneration.
• Ion Imbalances:
  • Extracellular $K^+$ ↑: ↓ Membrane excitability.
• Oxidative Stress:
  • Reactive Oxygen Species (ROS) ↑: Damage cell components.
• $Ca^{2+}$ Dysregulation:
  • Impaired SR $Ca^{2+}$ release/reuptake.

⭐ $P_i$ accumulation is a key factor in high-intensity exercise fatigue, directly inhibiting actin-myosin interaction and $Ca^{2+}$ release from SR.

High‑Yield Points - ⚡ Biggest Takeaways

• ATP is the immediate energy for muscle contraction.
• Creatine phosphate rapidly regenerates ATP for initial seconds of activity.
• Anaerobic glycolysis (2 ATP/glucose) fuels short, intense exercise, producing lactate.
• Aerobic metabolism (glucose, fatty acids) yields ~32 ATP/glucose for endurance.
• Muscle fatigue involves ↓ATP, ↑Pi, ↓pH, glycogen depletion, and K+ imbalance.
• Oxygen debt (EPOC) is post-exercise O₂ uptake to restore metabolic balance.
• Cori cycle: Lactate from muscle converted to glucose in the liver.