14.10
Muscle Recovery and Fatigue
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Q1: What causes muscle fatigue during intense physical activity?
During intense exercise, muscles demand more ATP than aerobic pathways can supply. Compressed blood vessels reduce oxygen delivery, forcing muscles to rely on anaerobic glycolysis. This produces lactic acid, and accumulated lactate and hydrogen ions lower cellular pH, causing lactic acidosis. This acidic environment prevents muscle contractions, resulting in fatigue.
Q2: How does the body recover ATP after muscle fatigue?
Recovery requires increased oxygen availability. The enzyme lactate dehydrogenase converts accumulated lactate back to pyruvate. Mitochondria use pyruvate to generate ATP or convert it to glycogen for storage. Creatine kinase then uses surplus ATP to rebuild phosphocreatine reserves, restoring the muscle's energy capacity.
Q3: What role does the Cori cycle play in muscle recovery?
The Cori cycle transfers excess lactate from fatigued muscles to the liver, where it converts back to glucose. This glucose returns to muscle tissue to rebuild glycogen stores or produce ATP. This cyclic metabolic pathway ensures efficient lactate metabolism and sustains energy production during recovery.
Q4: Is muscle fatigue only caused by lactic acid accumulation?
No. While lactic acidosis is a primary mechanism, fatigue involves multiple factors. Central fatigue occurs when the central nervous system signals the need to stop activity, serving as a protective response. This prevents muscle damage from overuse, suggesting fatigue has both peripheral and neural components.
Q5: Why do compressed blood vessels impair muscle oxygen supply during exercise?
Vigorous muscle contractions compress blood vessels within the tissue, restricting blood flow and oxygen delivery. This creates low-oxygen conditions that force muscles to switch from efficient aerobic ATP production to anaerobic glycolysis, which generates lactate and contributes to fatigue development.
Q6: How does lactic acidosis prevent muscle contraction?
Accumulated lactate and hydrogen ions from ATP hydrolysis lower cellular pH, creating lactic acidosis. This acidic environment disrupts the chemical processes required for muscle contraction, effectively halting force generation. Restoring normal pH through oxygen-dependent lactate clearance is essential for muscle function recovery.
Q7: What happens to pyruvate after lactate dehydrogenase converts lactate?
Pyruvate enters two pathways depending on energy needs. Mitochondria oxidize pyruvate to generate ATP through aerobic respiration. Alternatively, pyruvate undergoes enzymatic conversion to glycogen for storage, replenishing muscle fuel reserves depleted during intense activity and supporting sustained muscle performance recovery.