Physical activity among children and teens is lower than previously thought, according to a new study which found that 19-year-olds are as sedentary as 60- year-olds.
The findings come amid heightened concern that exercise deficits are contributing to the growing obesity epidemic, particularly among children and teens, researchers said.
The study by Johns Hopkins Bloomberg School of Public Health in the US found that young adults after the age of 20 show the only increases in activity over the lifespan, and starting at age 35, activity levels declined through midlife and older adulthood.
It also identified different times throughout the day when activity was highest and lowest, across age groups and between males and females.
These patterns, the researchers said, could inform programmes aimed at increasing physical activity by targeting not only age groups but times with the least activity, such as during the morning for children and adolescents.
"Activity levels at the end of adolescence were alarmingly low, and by age 19, they were comparable to 60- year-olds," said Assistant Professor Vadim Zipunnikov.
"For school-age children, the primary window for activity was the afternoon between 2 and 6 pm. So the big question is how do we modify daily schedules, in schools for example, to be more conducive to increasing physical activity?" said Zipunnikov, senior author of the study published in the journal Preventive Medicine.
The researchers used data from the US National Health and Nutrition Examination Survey from the 2003-2004 and 2005-2006 survey cycles.
The 12,529 participants wore tracking devices for seven straight days, removing them for only bathing and at bedtime. The devices measured how much time participants were sedentary or engaged in light or moderate-to-vigorous physical activity.
Activity among 20-somethings, the only age group that saw an increase in activity levels, was spread out throughout the day, with an increase in physical activity in the early morning, compared to younger adolescents, researchers said.
The increase may be related to starting full-time work and other life transitions.
For all age groups, males generally had higher activity levels than females, particularly high-intensity activity, but after midlife, these levels dropped off sharply compared to females.
Among adults 60 years and older, males were more sedentary and had lower light-intensity activity levels than females.
The study confirmed that recommended guidelines were not being met. For instance, the World Health Organisation (WHO) recommends at least 60 minutes of moderate-to-vigorous physical activity a day for children aged five to 17 years.
The study found that more than 25 per cent of boys and 50 per cent of girls aged six to 11 and over 50 per cent of male and 75 per cent of female adolescents aged 12 to 19 had not met the WHO recommendation.