VO2max is the gold standard measure of cardiorespiratory fitness. In a highly trained endurance athlete, which is the PRIMARY limiting factor for VO2max?

• A Skeletal muscle oxidative enzyme capacity — mitochondrial density limits O2 utilization
• B Lung diffusion capacity — alveolar O2 diffusion limits arterial saturation at maximal exercise
• C Blood hemoglobin concentration — O2 carrying capacity determines VO2max
• D Maximal cardiac output (stroke volume × heart rate) — O2 delivery to working muscles is the central limiting factor in normal healthy subjects ✓

Explanation

In trained athletes at sea level, VO2max is primarily limited by the central cardiovascular system — specifically, maximal cardiac output (CO = SV × HR). This determines O2 delivery (DO2 = CO × [O2]arterial). Training increases stroke volume (eccentric LV hypertrophy, increased EDV — athlete's heart) and lowers resting HR, allowing very high maximal CO (up to 35–40 L/min in elite athletes vs. 20–25 L/min in untrained). Peripheral factors (mitochondrial density, capillary density, oxidative enzymes) improve with training too, but central CO delivery is the rate-limiting step. At extreme altitude, diffusion limitation becomes relevant, but not at sea level in athletes.

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

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