Muscles are the ultimate self-tuning organs. If you rely on them to perform daily labor, they respond by growing bigger and stronger. But if you don’t give them anything useful to do, they shrink. That way, your body saves on the metabolic cost of keeping them alive. After all, the more muscles you have, the more calories you burn, even at rest—and your frugal body isn’t willing to waste all that valuable fuel.
In the modern world, where the hardest labor many people do is to reach across the sofa for the TV remote, our daily activity just isn’t enough to keep our muscles healthy. To fill the gap, we created exercise—basically, a practice in which we lift heavy things and then put them down in exactly the same place, or run like crazy on a machine without actually going anywhere.
Exercise is a thoroughly modern invention. Thousands of years ago, the balance between rest and activity was almost exactly the reverse of what it is today. People spent most of their lives straining their bodies, and if they had a moment of free time, the healthiest thing they could do was rest their weary muscles. Today, we spend most of our lives sitting in one place and thinking hard (or at least trying to look like we’re thinking hard). When we have time off, we use exercise to build and maintain healthy muscles— or, from a more cynical perspective, to give the illusion that our bodies are being put to good use. But if you want to spend your twilight years strong, hearty, and with all the muscular strength you need to pull your bottom off a toilet seat, you need a regular regimen of exercise.
As you probably know, there are two basic types of exercise:
• Aerobic exercise. This is the heart-pounding, fast-breathing sort of exercise you perform when jogging, swimming, cycling, or jumping on a trampoline. A regular regimen of aerobic exercise strengthens your heart, improves your lung capacity, and increases your endurance. It has a cascade of other potentially beneficial effects on the body, too—for example, it can improve your coordination, boost your metabolism, and burn fat. However, aerobic exercise isn’t the best way to build your muscles.
• Strength training. This is the intense muscle-straining exercise you perform when lifting weights or doing sit-ups. It makes your muscles contract much more forcefully, but for much shorter periods of time. Although a regular regimen of strength training won’t improve your stamina, it will pump up your muscles.
Both types of exercise are important, and their benefits are complementary. You’ll learn more about aerobic exercise on page 172, when you explore the heart. In the following sections, you’ll learn more about how muscles develop, and you’ll get some practical tips to help with your own strength training.
Source of Information : Oreilly - Your Body Missing Manual