Strong muscular function is the clearest
requirement for exercise, and enhanced muscular ability is the most obvious
benefit of regular exercise. Obvious or not, there is extra to your muscles
than meets the eye.
Similar to all living tissue, muscle cells
require oxygen; they get it from a rich network of tiny blood vessels called
capillaries. They also need power which is produced by thousands of tiny
factories called mitochondria. The mitochondria are packed with enzymes that
convert glucose (sugar) from the blood into energy; they can also generate
energy from glycogen (a starch) stored right in the muscles themselves, and
they can burn fat.
Muscles improve with regular use, but the
type of exercise you perform determines the type of improvement. Exercises such
as walking or biking increase your muscles' blood supply, energy stores, and
mitochondrial activity. The result is better oxygen uptake and a much more
efficient metabolism; you'll recognize it as improved endurance. In contrast,
exercises such as weight lifting increase the size and power of individual
muscle cells, increasing the bulk and strength of your muscles. Whereas your
circulation and metabolism will benefit when you use any part of your body,
individual muscles improve only when they are put to work. In addition, as
muscles become stronger, they get shorter and tighter. You can overcome both
problems by planning a balanced program that includes regular stretching.
When you exercise, your breathing gets
deeper and faster. If you push yourself to the limit, you'll be panting and
gasping for air. As you get into shape, you'll be able to do much more exercise
with less respiratory effort—you'll "get your wind." Surprisingly,
though, your lungs don't deserve any of the credit. In fact, the lung is one of
the few human organs that does not improve with exercise training. The reason:
your lungs have such a large excess capacity that they do not have to improve
to meet the demands of exercise. In fact, you could get along perfectly well
with just one lung, even getting enough oxygen to permit vigorous exercise. Intense
exercise produces breathlessness because tissues aren't getting enough oxygen
and they are producing excessive amounts of carbon dioxide and acid. Regular
exercise improves your wind without changing your lungs because it boosts your
heart's ability to pump oxygen-rich blood and it enhances your muscles' ability
to extract and use that oxygen.
Your bones are much more than passive
supporting structures like the girders on a high-rise. Your body's 206 bones
are metallically active living tissues. Even after you've stopped growing, your
bones are constantly reconstructing themselves by resorbing old bone and
forming new tissue. At any one time, about seven percent of your body's bone is
being remodeled.
During youth, bone formation outpaces bone
resorption— that's how we grow. In our twenties and thirties, the two processes
are balanced and bones are at their strongest, containing about two and a half
pounds of calcium in the average adult. But beyond age forty or so, bone tissue
is removed faster than it is restored; in particular, menopause accelerates the
net loss of bone calcium in women. In about thirty-four million Americans, the
result is osteopenia (low bone calcium), and in another ten million, the result
is osteoporosis, a potentially debilitating disorder which is characterized by
thin, brittle bones that tend to fracture quite easily. You can help keep your
bones strong by getting enough calcium and vitamin D in your diet and by
staying away from tobacco and excessive amounts of alcohol. And exercise helps
by slowing the rate of bone resorption. But to strengthen your bones, you'll
need special types of exercises, weight-bearing and/or resistance exercise.
