Walking may not be enough on its own to produce significant health benefits, research suggests.
A team from Canada's University of Alberta compared a 10,000-step exercise programme with a more traditional fitness regime of moderate intensity.
Researchers found improvements in fitness levels were significantly higher in the second group.
They told an American College of Sports Medicine meeting that gentle exercise was not enough to get fit.
"You've got to do more than light exercise"
Lead researcher Dr Vicki Harber said: "Generally, low-intensity activity such as walking alone is not likely to give anybody marked health benefits compared to programmes that occasionally elevate the intensity."
Dr Harber and her colleagues were concerned there was too much focus on simply getting people to take exercise, rather than on its intensity.
They compared people on a 10,000-step exercise regime, which they completed at their own pace, with a group whose routine was tougher, but which left them enough breath to be able to speak one or two sentences with ease at the end.
Both routines, which lasted for six months, burned off the same amount of energy. In total 128 sedentary people took part in the project.
Fitness measures
The researchers assessed impact on fitness by measuring blood pressure, and peak oxygen uptake, a measure of lung capacity.
They found that the step programme increased peak oxygen uptake by an average of 4% over the six months - but the figure for the moderate intensity exercise group was 10%.
Other markers of overall health, such as fasting plasma glucose levels and blood fat levels were unaffected by either exercise programme.
Dr Harber said: "Our concern is that people might think what matters most is the total number of daily steps accumulated, and not pay much attention to the pace or effort invested in taking those steps."
She said the 10,000-step programme did help to get people motivated - and was an excellent way to start taking exercise.
"But to increase the effectiveness, one must add some intensity or "huff and puff" to their exercise.
"Across your day, while you are achieving those 10,000 steps, take 200 to 400 of them at a brisker pace.
'Educated guess'
"You've got to do more than light exercise and move towards the inclusion of regular moderate activity, and don't be shy to interject an occasional period of time at the vigorous level."
Professor Stuart Biddle, an expert in exercise science at the University of Loughborough, said it was possible that the current guidelines on how much exercise to take were set too low.
"They are based on a little bit of an educated guess," he said.
"However, you have got to strike a compromise between physiology and psychology. The harder you make it, the fewer people will actually do it.
"It may be that very small changes to the fitness of a large section of the population would have quite a big impact."
Professor Biddle said there was no doubt that vigorous exercise was the way to get fit, but volume rather than intensity might be more useful in tackling issues such as obesity.
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