"Walking is man's best medicine." -- Hippocrates

Robert Sweetgall walked out of his house in Newark, Del., on Sept 7, 1984 to trek through 50 states in 50 weeks. Twenty million footsteps, 19 blisters and three pairs of shoes later he stepped into New York City, ending one of the most extensive studies of walking ever conducted.

His goal: "To tell Americans about the dangers of smoking, overeating and sedentary behavior, and also about the joy and real value of walking for exercise."

The 11,600-mile solo trek was the second long walk that Sweetgall has taken in three years and an outgrowth of his concern about the risks of heart disease and declining physical fitness.

"I lost my father to a heart attack in mid-1970s and in that same year, on three consecutive Sundays, I lost two uncles and an aunt to heart attacks," Sweetgall, 37, said as he passed through Washington last July after 9,820 miles. "So I've seen the ill effects. Going back to my childhood, I never was a gifted athlete. I graduated from college 25-30 pounds overweight and started when I got a job as a chemical engineer at Dupont to get into a fitness program, losing weight slowly and slowing, picking up jogging and other sports. I saw the benefits of a fitness program."

Sweetgall resigned after 12 years at Dupont in 1981 to devote himself to wellness promotion and create the Foundation for the Development of Cardiovascular Health, a public charity that organizes educational and research programs to improve awareness of life style risk factors related to heart disease. The foundation's first program was the Run for American Youth in 1982; there, Sweetgall began his first long walk, a 10,608-mile crusade around the perimeter of the United States -- to warn Americans of the risks of heart disease.

The surge in exercise walking comes on the heels of the running boom, which triggered mass awareness of physical fitness. Recent studies have found that walking has become the country's number one exercise and participation sport. Rippe, director of the exercise physiology laboratory and medical director of cardiac rehabilitation at the University of Massachusetts Medical Center at Worcester, estimates that 55 million Americans walk for exercise, an increase of 18 million since the '70s, and that number is expected to soar to 80 million by the end of this decade. The latest U.S. Census reported that 99 million Americans walk for pleasure.

Oliver Wendell Holmes summed up the pleasures of walking this way: "In walking, the will and the muscles are so accustomed to work together and perform their task with so little expenditure of force that the intellect is left comparatively free."

This is one of the major reasons walking has become such a popular form of exercise. But there are many others. Walking is convenient: you can walk almost every day, in any type of terrain, and incorporate it into any life style. Walking is inexpensive: you don't need elaborate equipment, just a good pair of walking shoes. Walking is injury-free: it conditions the muscles and lungs, yet is gentle on the body. Since walkers keep one foot on the ground at all times, they land with only 1 1/2 times their body weight on their joints, compared with the 3 1/2 times the body weight that runners and aerobic dancers place on their joints, according to Dr. James M. Rippe, director of exercise physiology laboratory at the University of Massachusetts at Worcester.

Walking confers the same cardiovascular benefits as running, said Gary D. Yanker, author of "The Complete Book of Exercisewalking," although it takes longer to walk and accrue benefits than it does to run. Walking briskly for one mile may take about 15 minutes and burn nearly as many calories as running the same distance in about half that time.

The response to Sweetgall's first trip was so great that he decided to try it again. But his second journey took on added significance. For 363 days starting Sept. 7, 1984, Sweetgall played guinea pig to an extensive research project that dramatizes the effects of exercise on health.

Said Rippe, co-director of the research: "Rob's walk is just the tip of the iceberg."

The research on Sweetgall further outlines the cardiovascular benefits of walking. During his cross-country trek, Sweetgall was flown back to the U-Mass. every four to six weeks for monitering of physiologic changes that might be related to his walking.

Rippe and Frank Katch, chairman of the department of exercise science at the university, gauged Sweetgall's resting metabolic rate, body composition, diet, cardiovascular fitness, pulmonary function, musculoskeletal system, blood composition and cardiac size and function.

The results of Sweetgall's walk confirmed studies of other less-traveled walkers that Rippe and Katch perform in their laboratories.

Highlights of the preliminary findings, released earlier this month, showed that Sweetgall experienced these changes:

*Oxygen consumption capacity increased. Even after avoiding exercise for two months prior to the walk, Sweetgall began his walk with a capacity to consume about 20 percent more oxygen than the average 37-year-old male.

After six weeks of walking, which was equivalent to a year of a normal person's walking three to four miles a day, he gained an additional 10 percent. He had a slight decline in oxygen capacity at the end of the trip, due primarily to fatigue.

*Cardiovascular condition improved.

When tested at levels slower than a normal stress test -- at normal walking pace of 3 mph to 5 mph -- Sweetgall's heart rate and oxygen consumption decreased over the course of the year as he became conditioned.

*No injuries. Although Sweetgall walked more than 11,000 miles, he didn't miss a single day because of an orthopedic injury. When filmed at high speeds, no major orthopedic problem or biomechanical breakdown was evident.

"We know that brisk walking, walking at speeds in excess of 4 mph for the average individual, is an excellent way of using the large muscle groups in repetitive fashion in getting aerobic exercise," Rippe said.

"This is the kind of exercise that increases your capacity to utilize oxygen."

In general, Rippe said, walking is a very safe activity because it is less likely to cause a long-term injury. The one danger of walking, Rippe warned, is that people don't walk hard enough to get their heart beating at more than 80 percent of their target rate, which is essential to receiving full cardiovascular benefits.