The adolescent years are when people's bodies are supposed to start the ascent to their physical peak. Teenagers are growing like beanstalks. Their hormones are raging. They're eager for new experiences. By all accounts, this should be among the most active periods in a person's lifetime.
Except it turns out it's not.
In an eye-opening study involving 12,529 Americans ages 6 to 85, researchers mapped how physical activity changes over a lifetime. The participants, part of the 2003-2004 and 2005-2006 cycles of the National Health and Nutrition Examination Survey, wore accelerometers, devices that measure movement, for seven consecutive days. For the purposes of the analysis, researchers counted all types of movement, not just exercise.
The first thing to note about the results, published in the August issue of the journal of Preventive Medicine, is that physical activity appears to be at its highest at age 6. If you've ever seen a squirmy kindergarten class that shouldn't be a surprise.
Vijay Varma, a National Institute on Aging researcher and lead author of the study, said that there has been a belief that physical activity gradually declines across the entire life span. But according to the new data, there seems to be a sharper-than-expected decline during childhood — starting in elementary school and continuing through middle school and high school. By age 19, the average American is as sedentary as a 60-year-old.
“At 60-plus, many people have health issues that might cause a restriction in movement, but why is this happening at age 19? It suggests that the social structures in place may not be supporting physical activity,” Varma said in an interview.
He theorized that the modern school day, which requires sitting for large amounts of time and where recess is often compressed into 20 to 30 minutes a day, may be partly to blame. There's also the issue of early school-bell times, which researchers have found lead to sleep deprivation.
“The timing of school isn’t consistent with biology of when kids wake up and go to sleep,” he explained.
Varma and co-author Vadim Zipunnikov, assistant professor at the Johns Hopkins Bloomberg School of Public Health, point out that the data shows that school-age children were the most active between 2 and 6 p.m., or after school.
Another reason for the sedentary day is likely to be screen time. Studies about how long we spend parked in front of our TVs, laptops, tablets and phones tend to become outdated quickly because of the constant rollout of new technology, but the numbers have been consistently high — as much as seven to nine hours per day. While the American Academy of Pediatrics recently loosened its recommendations for screen time, almost everyone agrees that too much of it leaves less time for physical activity, which can lead to a higher risk of obesity and depression.
Calling the end of adolescence a “high-risk time period for physical inactivity,” the study confirms that most children are not getting the minimum amount of activity — at least 60 minutes of a moderate-to-vigorous workout — recommended by the World Health Organization. Among 6-to-11-year-olds, 25 percent of boys and 50 percent of girls were not meeting the target. For adolescents ages 12 to 19, the situation was even more dire, with 50 percent of males and 75 percent of females falling short, the study found.
The next surprise in the study involves people in their 20s. The data show activity levels go up during this period — and this is important because this is the only period when people are moving more. Varma calls this a “catch-up” period and believes this, too, may be related to social factors. While the increase in activity was spread throughout the day, there was a noticeable spike in the early morning as compared to teenagers. According to the study, “emerging adulthood represent a period of multiple life transitions, including initiation of full-time work, increased household responsibilities, and changes in family structure including marriage and becoming a parent.”
As expected, physical activity starts to decline at around age 35, and that trend continues through midlife and beyond. That's consistent with previous studies and attributed to the wear and tear on our bodies as we age.
The timing of physical activity showed that as children age, their physical activity moves later and later in the day until it flips after age 19 to more activity in the mornings.
“These findings broadly suggest to us we really need to start looking at when individuals are being more active so we can home in on what is occurring,” Varma said, “and start to design physical activity interventions that might target those behaviors.”