Questions: Muscle Metabolism and Fatigue

Question 1

During a 400m sprint, which substances are primarily responsible for the burning sensation and declining force output in the final stretch?

• A. Lactic acid accumulation in muscle fibers
• B. Complete depletion of all ATP stores
• C. Accumulation of inorganic phosphate and H⁺ ions
• D. Failure of motor neurons to fire

Answer: (C)

Inorganic phosphate (Pi, released as phosphocreatine and ATP break down) and hydrogen ions (H⁺, from ATP hydrolysis) are the primary culprits: Pi impairs calcium release from the sarcoplasmic reticulum and inhibits cross-bridge cycling, while H⁺ reduces the calcium sensitivity of troponin. Lactate is a common scapegoat but is actually recycled as fuel and is not the direct cause of fatigue.

Question 2

Type I (slow-oxidative) muscle fibers fatigue more quickly than Type II (fast-glycolytic) fibers because they generate less force per contraction.

• T. True
• F. False

Answer: False

Type I fibers are fatigue-resistant, not fatigue-prone. They generate less peak force than Type II fibers but are densely packed with mitochondria and myoglobin, making them highly efficient for sustained aerobic effort. Type II fibers produce more force rapidly but rely more on anaerobic glycolysis and accumulate fatigue-inducing metabolites quickly.

Question 3

Why is the phosphocreatine (PCr) system used at the very start of intense exercise, even though aerobic oxidation yields far more ATP per glucose molecule?

Model answer: The PCr system regenerates ATP almost instantaneously through a single enzymatic reaction requiring no oxygen and no complex metabolic cascade. Aerobic oxidation, despite its higher yield, takes several seconds to ramp up because it requires increased oxygen delivery, mitochondrial enzyme activation, and substrate mobilization. When ATP is needed immediately, speed of regeneration matters more than efficiency.

Energy systems are selected by rate of ATP demand, not total yield. PCr transfers a phosphate to ADP in milliseconds with a single enzyme (creatine kinase). Aerobic oxidation involves dozens of reactions across glycolysis, the citric acid cycle, and the electron transport chain — efficient but inherently slower to activate.