The coronary arteries fill only when the heart muscle is relaxed (during diastole). A slower heart rate means that the heart is relaxed for a longer period of time and thus has more time to fill the normal or partially obstructed coronary arteries. Therefore, a better supply of and other nutrients reaches the heart muscle. A fit heart idles at a lower speed and does not strain during maximal activity. The difference between the unfit and fit heart is similar to the performance of a poorly tuned 1934 car as opposed to a new 1996 model. Although frequent exercise may slightly lower your heart rate at rest and for a given exercise, there is lack of scientific evidence that this effect can prevent fatal or non fatal heart attacks. The slower heart rate does not prevent clot formation or atherosclerosis.
During exercise the requirement for oxygen by exercis¬ing muscle causes a lack of oxygen. Therefore, the rate at which you breathe increases from about 12 breaths per minute to 24–30 per minute, so that more oxygen is taken into the lungs and given up to the blood. Also during exercise resting muscles extract only about 30% of the oxygen from the blood bathing the muscle cells. Vigorously exercising muscle can extract over 75% of the circulating oxygen. The amount of oxygen extracted at peak exercise is your maximal oxygen uptake and reflects the limit of your endurance or cardiopulmonary (cardio¬vascular) conditioning. An increase in your maximal oxygen uptake is the adaptation of the body to aerobic exercise. The body makes more efficient use of the oxygen available, and there is also less work for the heart. It is this physiological adaptation that allows you to have more stamina and less fatigue at a given level of aerobic exercise (Fig. 2).

Страницы: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12