Its been six years since the federal government in the United States published exercise guidelines for adults. Straightforward and flexible, they recommend that adults complete 150 minutes of moderate or 75 minutes of vigorous aerobic exercise each week. Either prolonged sessions or multiple, shorter workouts are OK, and the form is never specified.
Since then, the governments of many other nations, including Canada and Britain, have endorsed essentially the same guidelines for their citizens, and doctors now routinely advise their out-of-shape patients to begin exercising moderately.
But no scientific studies had determined whether average people know what the recommended intensities feel like in action. So for a study published last month in PLOS One, researchers at York University in Toronto recruited 129 sedentary adult Canadians aged 18 to 64 and set out to see what they knew about exercising for health.
The formal exercise guidelines do offer guidance for determining the intensity of your workout. You can use heart rate, for instance. During moderate exercise, according to the Canadian guidelines, your pulse should rise to about 64 per cent to 76 per cent of your maximum heart rate; during vigorous exercise, your pulse should hover between about 77 per cent and 90 per cent of your maximum. More casually, the American guidelines suggest that during moderate exercise, you should be able to talk, but not sing, while during vigorous activity, you will not be able to say more than a few words without pausing for a breath.
The Canadian researchers began by asking volunteers if they were familiar with the national exercise guidelines. A few said they were, although most were not. So the researchers handed out copies of the guidelines for them to study, and then asked if they felt that they understood the guidelines.
With surprisingly little demur, almost all of the volunteers said that the guidelines were clear and they felt confident that they could complete the requisite amounts of moderate exercise. Quite a few of the volunteers said that they believed that they indeed were already meeting the guidelines.
The scientists then measured the volunteers actual maximum heart rate with a treadmill test before having them walk or jog on the treadmill at a pace that they felt to be alternately light, moderate and vigorous. The volunteers were asked to maintain each of the desired intensities for about three minutes, so that the researchers could track their heart rates. Finally, the volunteers were asked to walk at the slowest pace they felt would qualify as moderate, meaning the slowest pace at which someone could expect to gain health benefits from the exercise.
The volunteers were, as it turned out, quite inept at judging intensity. Few maintained a heart rate above 65 per cent of their maximum when they were supposedly exercising moderately; even fewer reached a heart rate above 75 per cent of maximum during their version of vigorous exercise.
Perhaps most telling, most volunteers walked at a decidedly languorous pace when asked to estimate the lowest-intensity exercise that would qualify as moderate and provide robust health benefits. Only about 25 per cent reached a pace that raised their heart rate into the moderate range.
In general, during each of the tests, the volunteers overestimated how hard they were exercising, said Jennifer L Kuk, a professor of kinesiology at York University, who oversaw the experiment.
The implications of that finding are worrisome, she continued, on both a personal and public health level. At present, 15 per cent to 25 per cent of American and Canadian adults report in surveys that they exercise intensely enough to meet the national guidelines. But if many of them are exaggerating their efforts, however unintentionally, Dr Kuk said, the problem of physical inactivity may be even larger than the surveys suggest.
- Gretchen Reynold