The heart is a muscle that is required to contract continuously throughout your life to deliver oxygen to all organs in the body. Your lungs breathe in oxygen and breathe out carbon dioxide. Blood vessels connect the heart and lungs so that carbon dioxide can be removed from the blood and oxygen can be added to the blood. The heart then pumps this blood throughout the body. During exercise your heart must pump more often and more strongly to supply oxygen to your exercising muscles to make energy. In turn, you breathe in more often and more deeply to increase the amount of oxygen you inhale and carbon dioxide that you exhale.
Therefore, the basis of cardiorespiratory training is to place greater demands on the heart (e.g., make the heart beat more times per minute) than what is required during rest. By regularly overloading the heart in this fashion, it will become stronger. This results in pumping more blood and delivering more oxygen to the body per heart beat, and a lower resting heart rate.
How does lower resting heart rate affect aerobic capacity? Maximum heart rate is determined largely by genetics and age: view it as a fixed number of beats per minute. So, by lowering your resting heart rate, you increase the reserve capacity of your heart, or the number of beats between your resting and maximum heart rates. Therefore, if you decrease your resting heart rate by increasing your physical fitness, you will be able to perform more work above rest.
Since most daily activities are aerobic in nature, improving the delivery of oxygen to the muscles will improve your work performance. On the other hand, a sedentary lifestyle decreases the heart's ability to circulate blood and oxygen. So, view your heart as an aerobic muscle that must be conditioned for optimum functional health and fitness throughout your life. Even though some people are born with higher aerobic capacities than others, everyone will benefit from aerobic conditioning.
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