American College of Sports Medicine members are concerned official advice to do 30 minutes of gentle exercise each day is being misconstrued.
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Some may take this to include a mere stroll to the car, Circulation reports.
People should do at least 30 minutes of moderate exercise five days a week, or 20 minutes of vigorous exercise, like jogging, three days a week, they say.
There is confusion about what is the ideal amount and intensity of exercise to improve health.
All agree that regular exercise is essential. The World Health Organization has said 30 minutes of gentle exercise each day could be enough to sustain a minimum level of fitness.
Confusion
Recently, researchers at Queen's University, Belfast, found walking for half an hour on just three days a week gave similar fitness and blood pressure benefits to walking for 30 minutes five times a week.
The sports scientists, however, say this advice is misleading and could encourage people to do too little exercise.
"There are people who have not accepted, and others who have misinterpreted, the original recommendation.
"Some people continue to believe that only vigorous intensity activity will improve health while others believe that the light activities of their daily lives are sufficient to promote health," they told Circulation.
The authors include several experts who are on a high-level committee in the US which next year will announce America's new physical activity guidelines.
Their original recommendations in 1995 were quickly adopted by the WHO and by the UK government in 1996.
They now stress that adults need to top up their routine activities, such as casual walking and housework, with structured exercise.
This should include vigorous (jogging) and moderate aerobic exercise (a brisk walk), as well as twice-weekly activities, such as weight training, which maintain or increase muscular strength and endurance.
People can do short bouts of exercise to count towards their weekly goals, but these must last for at least 10 minutes.
They say that even more exercise than this may have further benefits. However, research has also shown that too much exercise can be damaging to the body.
Indeed, the sports scientists say that some people - including pregnant women and those aged 65 or older - might be advised to do slightly less exercise and modify the type of activities they do.
Professor Paul Gately, professor of exercise and obesity at Leeds Metropolitan University, told The Guardian that it was difficult to give "one-size-fits-all" advice.
"People who are very overweight would have to do an hour of exercise a day just to maintain their weight if they aren't going to change their diets," he said.
Dr David Haslam, of the National Obesity Forum, questioned whether it was realistic to expect people to do two weight training sessions each week.
"I'd rather see healthy habits built into daily life - gyms aren't a sustainable habit for all," he said.
A spokesman for the Department of Health said it was watching the developments but added there were no plans to change current advice.
