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Phys Ed Gretchen Reynolds on the science of fitness.

In an article under his byline for Sports Illustrated in December 1960, “The Soft American,” President-elect John F. Kennedy lamented the state of the nation’s fitness. As president he exhorted citizens to plunge into activities like 50-mile hikes.

As anyone sitting quietly and reading this article probably knows, that message did not resonate with most Americans. And these days, a majority get no planned exercise at all.

So at the recent annual meeting of the American College of Sports Medicine, one of the hottest topics was not how much exercise Americans should be completing, but how little.

Dozens of presentations and seminars examining a variety of activities concluded, essentially, that a few minutes of any strenuous exercise is sufficient to improve various measures of health and fitness.

“Everyone was talking” about those findings, said Linda S. Pescatello, a professor of kinesiology at the University of Connecticut, Storrs, who attended the conference, in Indianapolis. “It’s very appealing, obviously, the idea that you can get fit in a very short period of time.”

But she and other experts say there are still many unanswered questions about the long-term effects and efficacy of the wildly shrinking doses of exercise being studied and promoted by scientists and journalists, ( including this writer).

“People have been trying to figure out forever what the right amount of exercise is,” said Dr. Paul Thompson, a cardiologist at Hartford Hospital in Connecticut, who has long studied exercise.

In the past, formal recommendations have called for a substantial amount of regular exercise. For example, published guidelines from the Health and Human Services Department in 2008 suggested 150 minutes of moderate exercise per week — the equivalent of five 30-minute walks. The guidelines added that 75 minutes of vigorous exercise a week, like jogging, could be substituted.

These guidelines were based on a large body of science showing that 150 minutes of moderate exercise was associated with a longer life span and a reduced risk of heart disease, diabetes and other illnesses.

But in practical terms, the guidelines have not been a success. By most estimates, at least 80 percent of Americans don’t meet the recommendations. That has led to the quest to find a smaller amount of exercise that will produce health and fitness benefits without intimidating the millions who don’t work out.

And that, in turn, has resulted in the rise of interest in very brief, high-intensity interval training.

This approach to exercise started to take off in 2006, when Martin Gibala, a physiologist at McMaster University in Ontario, and his colleagues published a study showing that a three-minute sequence on an electronic stationary bicycle — 30 seconds of punishing, all-out pedaling followed by a brief rest, repeated five or six times — led to the same muscle-cell adaptations as 90 to 120 minutes of prolonged bike riding.

The study, which was published in The Journal of Physiology, soared to the top of the journal’s “most e-mailed” list and stayed there for years.

Since then, Dr. Gibala and his colleagues, as well as other groups of scientists, have been closely parsing the effects of brief bouts of intense exercise, trying to determine just what happens in the body when you work it very hard for a short period of time, and what dosage of such intense effort is likely to be most effective and tolerable for a majority of people.

The most recent research suggests that a few minutes per week of strenuous exercise can improve aerobic fitness, generally more quickly than moderate activity does.

In a representative study, which I wrote about this week, Norwegian scientists found that three four-minute runs a week — at a pace equivalent to 90 percent of a person’s maximal heart rate, an intensity that will feel, frankly, unpleasant — improved volunteers’ endurance capacity by about 10 percent after 10 weeks.

Other recent studies have shown that 16 to 30 minutes per week (depending on the study) of highly intense exercise also improves certain markers of health, with volunteers developing improved blood pressure and blood sugar levels after several weeks of these truncated workouts.

But so far, all the studies have been small, usually with only a few dozen volunteers, most of them men and often young. None have been longer than a few months.

“We know from some very good epidemiological studies,” said Dr. Thompson, “that 150 minutes of moderate exercise each week is clearly associated with improved health outcomes,” including longevity and reduced risk of many diseases. What we don’t know, he added, is whether that will be the case if people rely solely on a few minutes of intense exercise a week.

It’s particularly unclear whether short, hard workouts can help people maintain their weight. Weight maintenance means burning more calories than consumed, Dr. Thompson said, and “these short sessions do not result in much energy expenditure.”

Nor do they aid much in building muscle, Dr. Gibala said, adding that short, intense exercise “does not seem to stimulate the hypertrophic physiological pathways” that result in larger, stronger muscles.

What the new, abbreviated approach to exercise has going for it is brevity. In a 2011 study, eight male recreational runners in Britain reported preferring a workout of six three-minute intervals to one involving an easy 50-minute jog, because the interval session was soon over.

“It may not be the ideal form of exercise for everyone,” Dr. Gibala said. “And we have a lot more science to do.”

But he added: “I’m 45, with a family, and very busy,” and he has found the brief, intense sessions to be very helpful. So, he said, “this is how I work out now.”

Many scientists, in the United States and abroad, are conducting or planning additional studies of the effects of brief, intense training, Dr. Gibala said. But financing for large studies in this field is difficult to obtain, and results from long-term studies won’t, of course, be available for years.

For now, he says, if you’d like to try a high-intensity session, first visit a doctor for clearance, then simply push yourself very hard during your next workout, whether it is running, cycling or Zumba.

Researchers haven’t established a definitive period of time for an interval to provide maximum health benefits, Dr. Gibala said — although in his research and experience, a minute of hard effort followed by a minute of gentle recovery is effective.

Complete 10 such intervals three times a week for a total of 30 minutes of strenuous effort, he said, and “our data would indicate you’ll be in pretty good shape.”