Some people between the ages of 15 and 40, as well as 15% of North Americans over the age of 40, seek to exceed these achievements. Individuals who engage in regular vig¬orous exercise often reach a new ‘‘high,’’ and studies have shown that high levels of endorphins are produced in the bodies of such individuals. Endorphins are opiate-like chemicals similar to morphine that increase pain threshold and euphoria. With regular vigorous exercise, cardiovas¬cular conditioning is quickly achieved. A minimum frequency of three aerobic training bouts of exercise per week repeated for many weeks is a necessary requirement to produce training adaptations. Exercise continued daily for several weeks improves the ability of the heart to pump blood causing more blood to be ejected from the left ventricle into the aorta, which carries blood to all organs and tissues of the body. However, the heart muscle itself is not strengthened and coronary arteries do not feed the heart indefinitely with more blood. Unfortunately, exercise does not prevent the formation of atheroma or athero-matous obstruction of coronary arteries and has little effect on the prevention of angina or heart attacks.

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