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Thus, the normal untrained person can increase cardiac output a little over fourfold, and the well-trained athlete can increase output about sixfold. (Individual marathoners have been clocked at cardiac outputs as great as 35 to 40 L/min, seven to eight times normal resting output.)

Effect of Training on Heart Hypertrophy and on Cardiac Output.

From the foregoing data, it is clear that marathoners can achieve maximal cardiac outputs about 40 per cent greater than those achieved by untrained persons. This results mainly from the fact that the heart chambers of marathoners enlarge about 40 per cent; along with this enlargement of the chambers, the heart mass also increases 40 per cent or more. Therefore, not only do the skeletal muscles hypertrophy during athletic training but the heart does also. However, heart enlargement and increased pumping capacity occur almost entirely in the endurance types, not in the sprint types, of athletic training.

Even though the heart of the marathoner is considerably larger than that of the normal person, resting cardiac output is almost exactly the same as that in the normal person. However, this normal cardiac output is achieved by a large stroke volume at a reduced heart rate. Table 84-2 compares stroke volume and heart rate in the untrained person and the marathoner.

Thus, the heart-pumping effectiveness of each heartbeat is 40 to 50 per cent greater in the highly trained athlete than in the untrained person, but there is a corresponding decrease in heart rate at rest.

Role of Stroke Volume and Heart Rate in Increasing the Cardiac Output. Figure 84-10 shows the approximate changes in stroke volume and heart rate as the cardiac output increases from its resting level of about 5.5 L/min to 30 L/min in the marathon runner. The stroke volume increases from 105 to 162 milliliters, an increase of about 50 per cent, whereas the heart rate increases from 50 to 185 beats/min, an increase of 270 per cent. Therefore, the heart rate increase accounts by far for a greater

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