"Running isn't dangerous. People who die while running don't die from running; they die from heart disease," Dr. Kenneth Cooper said repeatedly more than a year ago when his close friend Jim Fixx died following a four-mile run in Vermont.

Cooper, who 17 years ago wrote the exercise bible of that era with his bestseller "Aerobics," again has taken a great deal of the available research, including studies performed at his Aerobics Center in Dallas, in an attempt to explain how and why Fixx died and how people who exercise can minimize the risk of heart attack during aerobic exercise.

Fixx's death touched off one of the most controversial debates in the history of exercise, and experts are still arguing: Did running kill Jim Fixx, or did running prolong the life of the 52-year-old author of "The Complete Book of Running"?

"The exercise antagonist will say that running shortened Jim Fixx's life," Cooper writes. "The exercise enthusiast, on the other hand, will argue that it enabled him to live nine years longer than his father."

In "Running Without Fear," Cooper describes in detail many of the factors that could have caused Fixx's death: He was greatly overweight and a heavy smoker before he began running at age 31, and his father had heart disease and died of a heart attack at age 43.

"Nothing known to man can totally protect him against coronary disease," Cooper writes, "but exercise remains an extremely important part of prevention."

Cooper provides a precise and simple explanation of the risks of heart attacks and sudden death -- including heredity, stressful life and personality, high blood pressure and cholesterol, diabetes, cigarette smoking and obesity -- before discussing the pros and cons of seven types of exercise: cross country skiing, running, swimming, indoor and outdoor cycling, walking and aerobic dancing.

Cooper reserves the second half of the 240-page book to explain in detail the use of a stress test to determine if one has heart disease and to discuss the extent of this nation's No. 1 killer.

His description of the method of stress testing and the analysis of the results of the tests is wonderfully informative and comprehendable. The accompanying depictions of the electrocardiogram (ECG), which records the electrical impulses of the heart, are helpful.

Cooper repeatedly suggests that he could have saved Fixx's life had he forced Fixx to take a stress test.

Cooper writes: "[Fixx] visited the Aerobics Center six months before his death to write a story on Johnny Kelley, the longtime Boston Marathoner. I invited Jim to undergo a stress test but he declined . . . for reasons known only to him.

"Unfortunately, if Jim Fixx had chosen to be tested, there is a very good chance that we would have noticed his coronary artery disease during the examination. Then, we could have prescribed treatment that might well have saved his life."

And the book ends with a provocative idea:

"As a follow-up to Jim Fixx's death, some exercise critics were saying that it's possible to be aerobically fit and still not be healthy. I'm sure that if Jim Fixx had been evaluated on a treadmill, he would have scored in the excellent or superior category for his sex and age. But was he really healthy?

"The answer is no, he wasn't healthy. Nor would I have classified him as such if, as I expect, his treadmill stress ECG had been grossly abnormal. So there's a difference between physical fitness and health . . ."