Fatigue Mechanisms

Fatigue Fundamentals - Defining the Drain

• Exercise-Induced Fatigue: Reversible ↓ in force/power output or task performance.
• Classification:
  • Central Fatigue: CNS origin (Brain, Spinal Cord).
    • Mechanism: ↓ neural drive to muscles; altered neurotransmission (e.g., serotonin, dopamine).
  • Peripheral Fatigue: Muscle/NMJ origin.
    • Sites: NMJ, sarcolemma, T-tubules, SR, contractile proteins.
    • Mechanism: Impaired muscle cell function (e.g., E-C coupling, ATP supply). vs Peripheral fatigue (NMJ/Muscle Fiber) sites diagram)

⭐ Central fatigue involves decreased voluntary activation of muscles, often influenced by psychological factors and brain neurotransmitter changes.

Brain's Burden - Central Command Quits

• Neurotransmitter Shifts:
  • ↑ Serotonin (5-HT)/Tryptophan ratio: Promotes fatigue. 📌 Serotonin SLOWS.
  • ↓ Dopamine: Reduces motivation/drive. 📌 Dopamine DRIVES.
  • Acetylcholine: Central roles beyond NMJ.
• BCAAs: Branched-Chain Amino Acids compete with tryptophan for brain entry, may ↓ central fatigue.
• Afferent Signals: Group III/IV muscle afferents signal metabolic stress (e.g., H⁺, K⁺) to CNS, inhibiting central motor drive.
• Energy Crisis: Hypoglycemia impairs CNS function, ↓ arousal & motor output.
• Psychological Factors: Perceived exertion (RPE), motivation, cognitive state modulate fatigue.

  ⭐ Central fatigue is significantly influenced by an increased brain serotonin (5-HT) to dopamine ratio, affecting mood and drive.

Muscle Muddle - Peripheral Power‑Failures

• Energy Substrate Depletion:
  • ATP-PCr: Rapidly depleted (<30s) in maximal intensity efforts. $PCr + ADP + H^+ \rightleftharpoons ATP + Cr$
  • Glycogen: Muscle stores vital for prolonged/high-intensity exercise; liver glycogen maintains blood glucose.
• Metabolic Byproduct Accumulation:
  • $H^+$ ions: From ATP hydrolysis & glycolysis. ↓pH; inhibits PFK, glycogen phosphorylase; ↓Ca²⁺ binding to troponin C.
  • $P_i$ (Inorganic Phosphate): From ATP/PCr breakdown. Impairs SR Ca²⁺ release; ↓myofibrillar Ca²⁺ sensitivity & cross-bridge force.
  • Lactate: Not direct fatigue cause; co-accumulates with $H^+$; can be a fuel source.
  • ROS (Reactive Oxygen Species): Exercise-induced; high levels impair enzyme function & E-C coupling.
• Excitation-Contraction (E-C) Coupling Failure:
  • NMJ (Neuromuscular Junction): ↓ACh release or sensitivity (rarely limiting in healthy individuals).
  • Sarcolemma excitability: Altered $Na^+/K^+$ pump activity; $K^+$ accumulation in T-tubules; impaired AP propagation.
  • SR $Ca^{2+}$ Handling: ↓$Ca^{2+}$ release (e.g., $P_i$, ROS effects); impaired $Ca^{2+}$ reuptake by SERCA.

⭐ $P_i$ accumulation is a key factor in fatigue during high-intensity exercise, directly impairing both SR Ca²⁺ release and cross-bridge force production.

Fatigue Factors & Fixes - Modulating Mechanisms

• Exercise Characteristics:
  • Intensity: High (↑$P_i$, ↓pH) vs. Prolonged (↓glycogen). Critical Power concept.
  • Duration: Longer → ↑substrate depletion, ↑dehydration, ↑hyperthermia.
  • Type: Endurance (central, metabolic) vs. Sprint/Strength (peripheral: $P_i$, $H^+$).
• Environmental Factors:
  • Heat Stress: ↑Glycogenolysis, ↑CV strain, ↑dehydration; hyperthermia → central fatigue.
  • Altitude (Hypoxia): ↓$O_2$ availability → impairs aerobic ATP, earlier fatigue.
• Individual Status:
  • Training: Adaptations (↑mitochondria, ↑capillarization, ↑glycogen, ↑buffering) delay fatigue.
  • Nutrition: Pre-exercise CHO & hydration levels vital. Ergogenic aids (e.g., caffeine, creatine) may aid.

⭐ Critical Power is the highest exercise intensity sustainable primarily by aerobic metabolism before fatigue rapidly ensues.

High‑Yield Points - ⚡ Biggest Takeaways

• Central fatigue: CNS-mediated ↓ neural drive to muscles, influenced by psychological factors.
• Peripheral fatigue: Impairment at the muscle fiber level, including E-C coupling and NMJ.
• Glycogen depletion: Critical in prolonged exercise, limits ATP resynthesis via glycolysis.
• H+ accumulation (lactic acidosis): Inhibits key enzymes (e.g., PFK) and Ca2+ binding to troponin.
• Pi accumulation (ATP hydrolysis): Impairs cross-bridge function (power stroke) & SR Ca2+ handling.
• K+ accumulation (extracellular): Depolarizes sarcolemma, reducing muscle fiber excitability and force output.