Nine out of ten high school students are not exercising enough to stay healthy and fit, setting up a pattern that often continues after they graduate, according to a new study published in the journal Pediatrics.
Researchers followed students at 44 high schools for four years, and found that only 9 percent met current exercise recommendations throughout that time. For the most part, those habits held steady after high school -- though college students were more active than non-students.
For students that continued to college, those living on campus exercised more than those living at home.
It's not clear why those students were more active. They might have been more involved in sports, for example, or simply walked more -- running from classes to dorms and other campus buildings, said lead researcher Kaigang Li.
"The walkability of your environment is important," said Li, an assistant professor of health and exercise science at Colorado State University, in Fort Collins.
This is not the first study to look at the physical condition of high school and college students. Several other studies have found that these two groups struggle with getting enough meaningful exercise.
According to Peter Katzmarzyk, a professor at Louisiana State University's Pennington Biomedical Research Center, in Baton Rouge, "This study really confirms the low levels of physical activity in adolescents, which appear to be maintained over time as they transition into young adulthood."
The strength of this study, he said, is that it objectively measured teens' activity levels: Students wore devices called accelerometers, which tracked how much they moved over the course of a week.
Katzmarzyk, who was not involved in the study, conducts research on child exercise patterns, obesity and health.
According to the U.S. Centers for Disease Control and Prevention (CDC), teenagers should get at least one hour of physical activity each day that includes exercise that boosts the heart rate, such as running. Kids should also try some strength-building activities -- for example, push-ups or lifting light-weights.
The CDC noted that a lack of physical education in U.S. schools may be a contributing factor in students’ understanding of exercise and how it can improve their health.
At one time, PE was a part of every student’s daily school activities, today, according to the CDC, only 29 percent of high school students have gym class every day.
The evidence from this new research and other studies makes a good argument for more physical education, according to Katzmarzyk.
"Any way that we can increase physical activity levels in adolescence might translate into maintaining higher levels of physical activity in young adulthood," he said. "So physical education in high school is certainly an important outlet for this."
Still, Li said, there are probably numerous reasons for teenagers' low exercise levels.
He noted that in elementary school, most U.S. kids do get enough physical activity. But there is a steep drop-off after that. According to Li, that could be related to many factors -- including heavier homework loads starting in middle school, and more time on cellphones and computers.
While schools and communities can advance opportunities for kids to be more physically fit, families that put a high priority on exercise and a healthy lifestyle give their children the ability to independently remain physically fit for a lifetime.