Muscle Blood Flow.

A key requirement of cardiovascular function in exercise is to deliver the required oxygen and other nutrients to the exercising muscles.(1) The actual contractile process itself temporarily decreases muscle blood flow because the contracting skeletal muscle compresses the intramuscular blood vessels; therefore, strong tonic muscle contractions can cause rapid muscle fatigue because of lack of delivery of enough oxygen and other nutrients during the continuous contraction. (2) The blood flow to muscles during exercise increases markedly.

Work Output, Oxygen Consumption, and Cardiac
Output During Exercise

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.)

Role of Stroke Volume and Heart Rate in Increasing
the Cardiac Output.

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 percent, whereas the heart rate increases from 50 to 185 beats/min,an increase of 270 percent. Therefore, the heart rate increase accounts by far for a greater proportion of the increase in cardiac output than does the increase in stroke volume during strenuous exercise.

Effect of Heart Disease and Old Age on Athletic
Performance

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 percent decrease between ages 18 and 80. Also, there is even more decrease in maximal breathing capacity.