28/10/2025
MUSCLE FATIGUE
Muscles fatigue when they can no longer generate the same amount of force or sustain activity, even though the effort continues. This happens because of several interacting physiological, biochemical, and neurological factors that disrupt how muscles produce and use energy.
Here’s a breakdown of the main causes:
⸻
🧠 1. Neural (Central) Fatigue
Fatigue can start in the nervous system, not just the muscle itself.
• Reduced motor drive: The brain and spinal cord may reduce the frequency or intensity of signals sent to muscles.
• Protective inhibition: The body “turns down” output to prevent damage when energy levels are too low or metabolic by-products are high.
• Neurotransmitter depletion: Low levels of chemicals like dopamine or acetylcholine can reduce signal transmission efficiency.
⸻
💪 2. Peripheral (Muscle) Fatigue
This occurs inside the muscle fibers themselves.
a. Energy depletion
• Muscles rely on ATP (adenosine triphosphate) for contraction.
• During intense or prolonged activity, stores of ATP, creatine phosphate, and glycogen become depleted.
• Without enough ATP, muscle fibers can’t sustain cross-bridge cycling (the mechanism that produces contraction).
b. Metabolic by-product accumulation
• Exercise generates hydrogen ions (H⁺), lactate, inorganic phosphate (Pi), and ADP.
• These alter muscle cell chemistry, reducing calcium release and sensitivity, and interfering with contraction.
• Contrary to popular belief, lactic acid itself isn’t the main culprit—it’s actually the acidic environment (low pH) that causes fatigue.
c. Calcium handling impairment
• The sarcoplasmic reticulum may release less calcium or fail to reuptake it efficiently.
• Calcium is essential for activating the actin–myosin interaction; if calcium regulation falters, force production drops.
⸻
❤️ 3. Circulatory and Oxygen Factors
• Reduced blood flow (especially in sustained contractions) limits oxygen and nutrient delivery.
• Less oxygen means a shift from aerobic to anaerobic metabolism, which produces fatigue-related by-products faster.
⸻
🔋 4. Recovery and Adaptation
Fatigue isn’t just failure—it’s a signal that the muscle needs rest and adaptation. After fatigue, the body:
• Replenishes glycogen and ATP stores
• Repairs microscopic damage to fibers
• Strengthens neuromuscular connections