A study from Harvard Medical School published in the New England Journal of Medicine shows that exercise prevents heart attacks in postmenopausal women and that those who spent the most time exercising had the greatest protection from heart attacks. The authors also concluded that greater intensity of exercise does not increase protection against heart attacks. This flies in the face of most well-done research.
Fitness refers to your heart. Your heart is a muscle. The only way to strengthen any muscle, including your heart, is to exercise that muscle against increasing resistance. When you exercise your legs, you contract your leg muscles, which squeeze the veins near them to pump blood form these veins toward your heart. When your leg muscles relax, the veins near them fill with blood. This alternate contracting and relaxing of your leg muscles squeezes and relaxes the veins near them to act like a second heart to push extra blood toward your heart. Your heart is muscular balloon. This extra blood pumped by your leg muscles fills the heart with more blood than it has at rest. The heart has to squeeze the blood inside its chambers and the more blood inside its chambers, the greater the resistance against the heart contractions. So the harder you exercise the greater the stretching and strengthening on the heart and the greater the gain in strength. So it is intensity that strengthens the heart, not how long you exercise.
Now for the strange conclusions of the study, that walking and vigorous exercise had the same reduction in heart attacks. The authors' own data shows that the faster a person walks, the less likely she is to suffer a heart attack. It is easy to compare slow and fast walking by measuring intensity and how many miles you walk per hour. It's not so easy to let a questionnaire decide how intensely a woman exercises. First of all, almost all of the women in the studies were not competitive athletes. What they call intense, I call junk miles. I wouldn't exercise that casually on my off days. Almost none of the women in the study were in organized exercise programs and almost none of these women have the foggiest idea of what vigorous exercise really means.
Doctors who do studies on exercise and are not exercisers themselves are unlikely to know the definition of vigorous exercise. The authors and the women in the study have probably never exercised vigorously enough to become short of breath and feel burning in their muscles. When they talk about intensity, it's the difference between crawling and almost crawling. Many other studies show that intensity determines fitness, and that duration is much less important. You can't be fit, have strong muscles, run fast, jump high, lift heavy, or throw far unless you stress and recover. The same goes for using exercise to prevent disease. Of course, people who have damaged hearts should not exercise intensely as it could harm them.
NEJM September 5, 2002
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