During maximal aerobic exercise, a trained athlete's cardiac output increases from 5 L/min at rest to 25 L/min. Which of the following best quantifies the relative contribution of heart rate vs. stroke volume to this increase, reflecting training adaptations?

• A HR increases from 60 to 190 bpm (×3.2); SV increases from 80 to 130 mL (×1.6); trained athletes have larger resting SV due to eccentric cardiac hypertrophy (athlete's heart), allowing greater SV at max exercise ✓
• B HR increases from 60 to 200 bpm (×3.3); SV remains constant at 80 mL; all cardiac output increase is from heart rate
• C HR increases from 50 to 190 bpm (×3.8); SV increases from 60 to 100 mL (×1.7); both changes are similar in trained and untrained individuals
• D HR increases from 70 to 140 bpm (×2.0); SV increases from 70 to 175 mL (×2.5); SV increase dominates because adrenergic stimulation primarily enhances inotropy in trained athletes

Explanation

In trained athletes, resting bradycardia (HR ~50–60 bpm) and elevated resting stroke volume (80–100 mL) result from eccentric cardiac hypertrophy (increased ventricular volume) and enhanced parasympathetic tone. During maximal exercise, HR rises to approximately 190–200 bpm and SV increases to 160–200 mL (due to increased venous return, reduced afterload, and adrenergic inotropy). In untrained individuals, max SV may reach only 100–120 mL. Option A is closest to textbook values showing both HR and SV contribute. Option B is incorrect (SV always increases with exercise). Option C understates the training difference. Option D overestimates relative SV contribution versus HR contribution, and the calculation 70 × 140 = 9,800 mL/min is far below 25 L/min.

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

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