SAN MATEO, Calif. — A computer-generated call featuring a friendly, recorded voice provided the same motivation as a live coach in boosting physical activity, according to a new study from Stanford University.

The results point the way to a cost-effective means of inspiring legions of inactive Americans to reap the benefits of exercise, while lowering the burden on the health care system from chronic diseases linked to excess weight and sedentary lifestyles.

“Physical activity is something you need to keep up throughout your life,’’ said Abby King, Ph.D., a senior investigator with the Stanford Prevention Research Center. King is the lead author of the study, published in this month’s issue of Health Psychology. The National Institute on Aging funded the research.

But changing entrenched habits among time-strapped adults is among the most vexing challenges facing health professionals seeking to make a dent in the enormous health toll of physical inactivity among Americans.

Only 30 percent of adults at middle age and beyond regularly work out, according to the Centers for Disease Control and Prevention. And 29 percent report they engage in no physical activity. Yet exercise is one of the best and cheapest antidotes to ailments related to aging, such as cardiovascular disease, hypertension and certain cancers.

So the Stanford researchers designed the first test to compare the long-term results of computer-generated calls based on pre-recorded messages from a professional female narrator with regular calls from a live health coach.

“The goal was to come up with interventions that were effective, and cost-effective,’’ said King.

The Stanford researchers recruited 218 Bay Area adults aged 55 and older who had been exercising less than one hour a week, and assigned them to one of three groups.

The first group received a 40-minute in-person meeting with a health educator, which included developing an exercise plan for reaching a goal of 30 minutes or more a day of moderate-intensity exercise usually a brisk walk most days of the week. After that, they received calls from the health coach during the 12-month study.

The second group got the same initial in-person consultation, but received follow-up calls from a computer-generated system, at the same frequency as the live coach.

The third group was offered weekly health education classes, which required in-class attendance, with no coaching.

Twelve months later, the groups receiving either live or computerized coaching had sharply increased their exercise levels, exceeding the U.S. Surgeon General’s guideline of achieving at least 150 minutes a week of physical activity.