What causes fatigue during high-intensity exercise due to lactate production?

• A. The presence of hydrogen ions increases acidity, slowing enzyme activity and glycogen breakdown, leading to fatigue.
• B. Lactate itself is the direct fatigue-inducing toxin.
• C. Fatigue is solely due to CO2 buildup.
• D. Fatigue is because oxygen is depleted.

Answer: (C)

During very intense effort the muscles rely on fast glycolysis to meet energy needs, which produces ATP quickly but also adds hydrogen ions to the muscle cells. The rise in acidity (lower pH) slows the enzymes that drive energy production and affects how well the contractile machinery works, so force output drops and fatigue sets in sooner. Lactate is produced as part of this glycolytic flux and can even be used as fuel later, but it’s the associated hydrogen ions driving the slowdown and fatigue, not lactate itself as a toxin. CO2 buildup and oxygen depletion play a role in metabolism and acid-base balance, but they are not the primary fatigue drivers in this high-intensity, short-duration context.