About 15%-20% of cardiac output goes to the skeletal muscles at rest. Cardiac output is equal to heart rate times stroke volume. Increasing or affecting either heart rate or stroke volume will affect the cardiac output. During exercise, the cardiac output normally will be increased three or four times compared to resting state due the increased heart rate and stroke volume. During exercise, the skeletal muscles will need more oxygen when muscle contraction, and it would activate the sympathetic nervous system to cause the heart to beat faster and also increase the stroke volume so that the maximum blood and oxygen could be delivered to the contracting muscles. Therefore, the increased in both heart rate and stroke volume will increase the cardiac
