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Exercise, even modest exercise, puts stress on nearly every part of your body. That sounds frightening, but it shouldn’t scare you away from exercise. In fact, if the stress of exercise is applied properly, nearly every part of your body will respond by growing stronger and healthier. The result is true fitness. It’s not measured by how fast you can run, how much you can lift, or how big your biceps are. Instead, real fitness is measured by how well your body can withstand stress of all sorts: the stress of exercise, the stress of disease, the psychosocial stresses of twenty-first-century life, and even the stress of the aging process. Exercise can make you fit and healthy. The trick is to know how to exercise properly and then to make it part of your daily life. And the way to start is by understanding how exercise affects your body.

Exercise and Your Body
Even the most committed couch potato has sprinted to catch a bus or an elevator, and all of us can remember how it feels to exercise. Physical exertion makes your heart beat faster and harder. Your breathing also gets faster and deeper. If you’re at it long enough, your skin will get flushed, warm, and damp with perspiration. Your muscles will be taut from effort, and they may ache and stiffen up for some time afterward. If you are really pushing your-self, you may notice some nausea, abdominal discomfort, or light headedness, and you might enjoy high spirits right after you come to a stop, only to feel tired, sleepy, or a bit grumpy later in the day. You don’t have to be an exercise physiologist to know that exercise makes your heart, lungs, and muscles work harder or that your metabolism speeds up, producing extra heat. But even though an occasional burst of exercise may enable you to catch a bus or enjoy a sporting afternoon with the kids, it won’t do much for your health. For fitness and health, sporadic exercise won’t do, but regular exercise will do very nicely indeed. The body responds to the stress of habitual exercise with a remarkable series of adaptations that are collectively known as the training effect.

Exercise and Your Heart
Your heart is incredibly strong, but exercise training will make it stronger and more durable. A healthy heart pumps about five quarts of blood a minute while you are resting quietly. When you dash to make that bus, your heart rate may double or even triple, and the remarkable little muscle will pump out up to twenty quarts of blood a minute. Diseased hearts can’t match this performance, but exercise-trained hearts can do much more. At maximum effort, an athlete’s heart can pump up to forty quarts of blood a minute, and it can sustain a high workload for much longer than the unconditioned heart can. How does regular exercise help your heart? Like your other muscles, your heart muscle gets larger and stronger with exercise. Exercise also makes the heart muscle more efficient, so it needs less oxygen for itself. Exercise training helps human hearts resist arrhythmias, including the abnormal pumping rhythms that can lead to sudden death. And moderate exercise will earn all of these heartfelt improvements for you.