ow strenuous does exercise need to be to protect against heart disease? If it feels strenuous, it will help, even if the workout falls short of recommended guidelines, according to a study released yesterday.

Debates over the relative merits of intense versus moderate exercise have gone on for years. The study's lead author, Dr. I-Min Lee, an epidemiologist at the Harvard School of Public Health, said the new research was meant to explore what worked and what didn't for older people and for those who are less capable of the kinds of intense exercise that standard guidelines often call for.

For instance, she said, walking three or four miles an hour for 30 minutes on most days of the week would meet the guidelines. "My grandmother is 93 and couldn't walk four miles an hour if her life depended on it," Dr. Lee said. "Two miles an hour is moderate or even vigorous exercise for her."

For the study, which was published in the journal Circulation, the researchers surveyed more than 7,000 men with an average age of 66 about how much they exercised and how hard they felt their workouts were.

Over the next seven years, men who described their exertion as "moderate" or "strong" were less likely to develop heart disease, even if their workouts fell short of levels called for in guidelines.

"If you can adhere to the recommendations, go ahead and do that," Dr. Lee said, because several studies show greater benefits from greater exertion.

"But for those who cannot, especially older people, don't be discouraged if you can't meet them," she continued. "If you walk or do another physical activity and find that it's hard, go ahead and do it. It will be beneficial."