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proportion of the increase in cardiac output than does the increase in stroke volume during strenuous exercise. The stroke volume normally reaches its maximum by the time the cardiac output has increased only halfway to its maximum. Any further increase in cardiac output must occur by increasing the heart rate.

Relation of Cardiovascular Performance to Vo2 Max. During maximal exercise, both the heart rate and the stroke volume are increased to about 95 per cent of their maximal levels. Because the cardiac output is equal to stroke volume times heart rate, one finds that the cardiac output is about 90 per cent of the maximum that the person can achieve. This is in contrast to about 65 per cent of maximum for pulmonary ventilation. Therefore, one can readily see that the cardiovascular system is normally much more limiting on Vo2 Max than is the respiratory system, because oxygen utilization by the body can never be more than the rate at which the cardiovascular system can transport oxygen to the tissues.

For this reason, it is frequently stated that the level of athletic performance that can be achieved by the marathoner mainly depends on the performance capability of his or her heart, because this is the most limiting link in the delivery of adequate oxygen to the exercising muscles. Therefore, the 40 per cent greater cardiac output that the marathoner can achieve over the average untrained male is probably the single most

Cardiac output (L/min)

Figure 84-10

Cardiac output (L/min)

Figure 84-10

Approximate stroke volume output and heart rate at different levels of cardiac output in a marathon athlete.

important physiologic benefit of the marathoner's training program.

Effect of Heart Disease and Old Age on Athletic Performance.

Because of the critical limitation that the cardiovascular system places on maximal performance in endurance athletics, one can readily understand that any type of heart disease that reduces maximal cardiac output will cause an almost corresponding decrease in achievable total body muscle power. Therefore, a person with congestive heart failure frequently has difficulty achieving even the muscle power required to climb out of bed, much less to walk across the floor.

The maximal cardiac output of older people also decreases considerably—there is as much as a 50 per cent decrease between age 18 and age 80. Also, there is even more decrease in maximal breathing capacity. For these reasons, as well as reduced skeletal muscle mass, the maximal achievable muscle power is greatly reduced in old age.

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