Fatigue mechanisms

Introduction to Fatigue - The Body's Fuse Box

• Definition: A reversible, exercise-induced decline in muscle force-generating capacity. A key protective mechanism, not simple exhaustion.
• Primary Sites:
  • Central Fatigue: Originates in the CNS, resulting in ↓ neural drive to the muscle.
  • Peripheral Fatigue: Arises from processes at or distal to the neuromuscular junction (NMJ).
• Protective Role: Prevents catastrophic ATP depletion and subsequent irreversible muscle damage (rigor).

⭐ Central fatigue often precedes significant peripheral fatigue, acting as an anticipatory governor to protect muscles from injury.

Peripheral Fatigue - Fuel Tank Empty

• Glycogen Depletion: Primary cause of fatigue in prolonged exercise (>60 min). Muscle glycogen is the main fuel for ATP synthesis during intense exercise. Depletion leads to "hitting the wall."
• Phosphocreatine (PCr) Depletion: Occurs rapidly during short, high-intensity bursts (<30s). Limits the rate of ATP regeneration ($PCr + ADP \leftrightarrow ATP + Cr$), reducing power output.
• Hypoglycemia: Depletion of liver glycogen can lead to a fall in blood glucose, impairing CNS function and contributing to fatigue.

⭐ Exam Favorite: The "crossover concept" dictates that as exercise intensity increases (above ~65% VO₂max), the body shifts from predominantly fat metabolism to carbohydrate metabolism for fuel, accelerating glycogen depletion.

Peripheral Fatigue - Ion Disarray Drama

• Action Potential (AP) Disruption: Intense muscle contraction causes ion shifts, primarily ↑ extracellular K⁺ from repeated repolarization.
  • This accumulation alters the resting membrane potential, impairing sarcolemma and T-tubule excitability.
  • Result: ↓ AP propagation, leading to ↓ Ca²⁺ release from the sarcoplasmic reticulum (SR).
• Calcium (Ca²⁺) Handling Failure:
  • SR Release & Reuptake: Impaired T-tubule APs directly ↓ Ca²⁺ release. SR Ca²⁺-ATPase (SERCA) pump activity also slows.
  • Sensitivity: Accumulated H⁺ and inorganic phosphate (Pi) compete with Ca²⁺, reducing troponin's binding sensitivity.

⭐ High-Yield: Elevated extracellular K⁺ can depolarize the muscle fiber membrane sufficiently to inactivate voltage-gated Na⁺ channels, directly hindering the excitability required for sustained contraction.

Central Fatigue - The Brain Says 'Nope'

• CNS-mediated reduction in neural drive to muscles, occurring upstream of the motor neuron, independent of peripheral muscle state.
• Key Mechanisms:
  • Neurotransmitter Flux: ↑ Brain Serotonin (5-HT) promotes lethargy. Altered dopamine/norepinephrine signaling reduces motivation and arousal.
  • Cerebral Energy Strain: Hypoglycemia can impair brain function and the perceived effort to continue exercise.
  • Psychological Override: Perception of effort and motivation can limit performance before true physiological failure.

⭐ The ↑ Serotonin/Dopamine ratio in the brain is strongly linked to feelings of tiredness and reduced motor drive during prolonged exercise.

High‑Yield Points - ⚡ Biggest Takeaways

• Central fatigue involves altered CNS neurotransmission, reducing the neural drive to exercising muscles.
• Peripheral fatigue arises from the motor unit, primarily due to metabolite accumulation and substrate depletion.
• Inorganic phosphate (Pi) accumulation is a key driver, directly impairing cross-bridge cycling and SR Ca²⁺ release.
• Glycogen depletion limits performance in prolonged endurance exercise ("hitting the wall").
• Phosphocreatine (PCr) depletion causes fatigue during short-duration, high-intensity bursts.
• Extracellular K⁺ accumulation can impair sarcolemma excitability.