Which statement about maximal heart rate and cardiac output across populations is true?

• A. Max heart rate is identical across individuals
• B. Trained individuals have lower stroke volume
• C. Max cardiac output is influenced by body size and training
• D. Age increases heart rate at maximum effort

Answer: (C)

Maximal cardiac output depends on two factors: how fast the heart can beat at maximum effort (HRmax) and how much blood it can eject with each beat at maximum effort (SVmax). HRmax is largely set by age and genetics and tends to decline with aging, while training primarily increases the amount of blood ejected per beat by boosting stroke volume. A bigger body typically has a larger heart and greater stroke volume, so body size can raise Qmax as well. Put together, maximal cardiac output is influenced by both body size and training because larger hearts deliver more blood per beat and training enhances the heart’s pumping capacity. The other statements don’t fit the physiology: max heart rate isn’t identical across individuals, endurance training increases stroke volume rather than decreasing it, and age does not increase heart rate at maximum effort.