Most American teenagers are not as active as they should be but a lack of exercise does not seem to be to blame for the rising rates of teen obesity, according to a US study.

Researchers, using government survey data from 1991 and 2007, found the amount of physical activity among US teens has not in fact changed significantly over the past two decades while the population, including children, has got heavier.

Researcher Youfa Wang, of Johns Hopkins University in Baltimore, said it came as a surprise to find that a lack of exercise was not to blame for the rise in obesity with nearly one-third of US children and teens now overweight or obese.

"Although only one third of US adolescents met the recommended levels of physical activity, there is no clear evidence they had become less active over the past decade while the prevalence of obesity continued to rise," said Wang.

He said there was no evidence that teens' exercise levels had changed appreciably at any time during the study period -- even though those years saw an increase in teen obesity.

For the study published in the journal Obesity Reviews, Wang and colleagues used data from an annual government survey that tracks the health and lifestyle of US high school students.

Overall they found only 35% of teenagers surveyed in 2007 met the current recommendations for physical activity — performing activities that gets the heart rate up at least one hour per day, five or more days out of the week.

But there was no evidence that teenagers' exercise habits shifted significantly during the study period.

In 1993, for example, 66% of teens got enough short bursts of vigorous exercise -- 20 minutes of running, biking or other heart-pumping activity at least three days per week. That figure was 64% in 2005.

When it came to moderate exercise which should, according to guidelines, be performed at least 30 minutes per day, on five or more days per week, only 27% met that goal in 1999.

That figure was unchanged in 2005.

On the other hand, there were some encouraging trends, according to Wang's team. One was the recent increase in time spent in school physical education classes.

In 2007, 30% of high schoolers were taking a daily physical education class compared to 25% in 1995.

The researchers also found a decline in teenagers' TV time. In 1999, 43% of students spent three or more hours watching TV on school days but this figure dipped to 35% in 2007.

Wang said these findings suggest that waning exercise levels are "not likely the major explanation of the recent increase in obesity among US adolescents."

He said other factors, like unhealthy diets, may be the driving force.

However, the researchers added that this does not mean it is fine for teenagers to be sedentary. Children and teens still need to develop regular exercise habits for the sake of their overall health, according to the researchers.

"Our study suggests that more vigorous efforts are needed to help young Americans engage in adequate regular physical activity and reduce sedentary behaviors, which will help promote good health," they said.