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What are the fitness benefits of running?
Cardiorespiratory fitness (aerobic fitness or "cardio") is the ability of your heart to pump stronger and more efficiently and your muscles to use oxygen more efficiently. As you get more aerobically fit, your heart will pump more blood and oxygen with each beat (this is called "stroke volume") and your muscles will extract (or consume) more oxygen. For instance, if you have 100 oxygen molecules floating around in your bloodstream, a conditioned muscle might consume 75 molecules, whereas a deconditioned muscle might only consume 30, or even fewer than that. In fact, elite distance runners can be as much as three times more efficient at consuming oxygen than sedentary individuals. Running improves your aerobic fitness by increasing the activity of enzymes and hormones that stimulate the muscles and the heart to work more efficiently.
What about running and burning fat?
For years, I've been asked if running burns more fat than other exercises. My hunch was that it might, but there was never any proof. In particular, I was always perplexed by the fact that swimming burns so many calories (in some cases even more than running), yet when you look at the physiques of Olympic swimmers and compare them to elite long-distance runners, you see a more defined, cut and leaner physique on the runner. Adjusting for something called self-selection, where individuals of a certain body type might select a specific sport (for example, lean people might choose long-distance running because they already have the body type for it), I never fully understood why swimmers and some other endurance athletes weren't quite as lean as runners. Then I read a study comparing fat burning in running and uphill walking to cycling and it turned out that fat burning was 28% higher during running and walking uphill than it was during cycling. The authors of the study aren't sure why this is so, but it is suggested that the pounding of weight-bearing activities like walking and running may cause more fat burning than a seated exercise like biking, or an activity like swimming where there is no pounding at all. This is intriguing research, but more needs to be done before we truly sort out these issues.
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