What's Going On in There?

The physiological effects of exercise on your body.

A month ago you could barely run two miles; your heart would be racing and your leg muscles would be sore. Now after running four times a week, a three-mile run is no sweat. What's going on inside your body?

Anyone who has exercised regularly has experienced the thrill of improving. You've improved, of course, because you've trained. But how exactly has your body adapted to the training? In what way have your muscles changed? What has happened to your heart? Why doesn't it beat as fast when you're in shape?

When you begin exercising regularly, your body undergoes several physiological and neuromuscular changes. Naturally the changes will vary according to the frequency duration and intensity of your training.

If you train for speed, you'll get faster. If you train for strength, you'll get stronger. Genetics also play a part in how your body adapts to training. Even if you and a friend follow the same training schedule, one of you may improve more quickly and dramatically than the other.

Remember that all of these changes take time - usually four to eight weeks -and all of the beneficial adaptions disappear when you stop training, usually faster than they developed.

How the Heart Adapts

The most important muscle that adapts to training is the heart. During exercise, it pumps blood containing oxygen, fluids and nutrients to the active muscles. Blood flow then drains the metabolic waste products away. The more blood pumped, the more oxygen is available to the exercising muscles. And as muscles train, they're better able to extract and use the oxygen to produce more work.

The heart adapts to aerobic exercise over time so it can pump more blood per stroke. In untrained people who exercise, cardiac output can increase to four times resting capacity. In the untrained female, it goes from pumping 4 to 5 liters a minute at rest to 16 to 20 liters a minute during exercise, primarily through an increase in heart rate.

What happens in trained athletes?

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Table of Contents

Foreword: Billie Jean King

Comments by Barb Harris
Editor in Chief,
Shape Magazine

General Health
Nutrition
Exercise
Common Medical Problems
Dental Health
Emergency!
Infectious Disease
Sexual Health
Emotional Well-Being
Eating Disorders
Alcohol & Other Drugs
Environmental Health
Prevention