Final answer:
Stroke volume is the component of cardiac output most responsive to training, as it increases with the heart's strength and efficiency gained through exercise.
Step-by-step explanation:
Of the two components of cardiac output (CO), stroke volume (SV) is most responsive to training. Cardiac output is calculated by multiplying stroke volume (the amount of blood pumped by each ventricle per beat) by heart rate (HR, in contractions per minute). As a result of endurance training, the heart muscle improves in efficiency, thereby enhancing the stroke volume; this is because exercise increases the size and mass of the heart, making it stronger and more efficient at pumping blood.
This enhancement in stroke volume leads to a greater cardiac reserve, which is the difference between maximum and resting cardiac output, and reflects the heart's capacity to pump blood. As an individual's cardiovascular fitness improves, their resting heart rate usually decreases, but it is the increase in stroke volume that allows for a significant boost in maximum cardiac output during exercise.