During incremental exercise testing, the anaerobic threshold is best identified by which of the following?
- A A nonlinear increase in minute ventilation relative to oxygen consumption (VCO2/VO2 inflection) ✓
- B A sustained rise in blood lactate above 4 mmol/L
- C The point at which heart rate reaches 85 percent of age-predicted maximum
- D The onset of metabolic acidosis with pH falling below 7.35
Correct answer: A. A nonlinear increase in minute ventilation relative to oxygen consumption (VCO2/VO2 inflection)
Explanation
The anaerobic threshold is identified noninvasively as the point where minute ventilation rises disproportionately to VO2 (ventilatory threshold), reflecting CO2 excess from bicarbonate buffering of lactate. Although lactate rises around this threshold, the 4 mmol/L fixed value (onset of blood lactate accumulation) is a separate concept. Heart rate percentage is an imprecise surrogate, and pH 7.35 is not the defining criterion.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
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