Muscle Fatigue Mechanisms

Expansion

Central fatigue

• Reduced motor cortical drive and motor unit recruitment
• Influenced by perceived effort, motivation, temperature, and altered brain serotonin and dopamine
• Demonstrated because superimposed electrical stimulation can produce more force than voluntary effort in a fatigued muscle

Peripheral fatigue

• Inorganic phosphate accumulation, impairing cross-bridge force and calcium release: probably the dominant factor in high-intensity work
• Impaired calcium release from the sarcoplasmic reticulum
• Glycogen depletion, the limiting factor in prolonged endurance exercise
• Potassium accumulation in the T tubules, reducing excitability
• Acidosis, which contributes less than traditionally taught

Note that lactate itself is now regarded as a fuel rather than a cause of fatigue, and it is not responsible for delayed onset muscle soreness, which reflects microscopic damage and inflammation peaking at 24 to 72 hours.

Different tasks fail for different reasons, which is why fatigue is task-specific.