During maximal aerobic exercise, VO₂max is reached when further increases in workload produce no additional increase in oxygen consumption. Which factor primarily limits VO₂max in a sedentary individual?

• A Pulmonary diffusion capacity, as alveolar O₂ transfer becomes diffusion-limited at high exercise intensities
• B Skeletal muscle mitochondrial density, which limits O₂ utilization despite adequate delivery
• C Hemoglobin concentration, as RBC transit time in pulmonary capillaries becomes too short for full saturation
• D Maximal cardiac output (oxygen delivery limitation), as the sedentary heart cannot sufficiently increase stroke volume or heart rate to deliver O₂ at the rate skeletal muscle can consume it ✓

Explanation

In sedentary individuals, VO₂max is primarily limited by central cardiovascular O₂ delivery—specifically maximal cardiac output (Fick principle: VO₂max = CO × A-VO₂ difference). Aerobic training increases maximal SV (cardiac hypertrophy, increased plasma volume) raising CO; the peripheral A-VO₂ difference is near-maximal even in untrained individuals. Pulmonary diffusion limitation occurs mainly in elite athletes at extreme intensities; mitochondrial density is the primary peripheral limit in trained subjects.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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