Researchers report that in the typical high school gym class -- where there are a few jumping jacks before a halfhearted game of softball -- students are active for an average of just 16 minutes.
The report by Cornell University researchers also found that adding 200 minutes more of physical education time per week had little effect.
"What's actually going on in gym classes? Is it a joke?" asked John Cawley, lead author of the study and a professor of policy analysis and management at Cornell.
The study was based on annual surveys of 37,000 high schoolers by the Centers for Disease Control and Prevention. The data come from the annual youth behavior surveys from 1999, 2001 and 2003, which include questions about students' exercise habits.
As obesity rates climb among children, the role of schools in shaping students' eating and exercise habits has come under intense scrutiny. In recent years, nearly all states have introduced bills to increase or reform physical education. Healthy People 2010, a federal initiative to improve physical fitness, has made it a goal to improve curriculum and diminish the amount of time students waste being sedentary in gym class.
The National Association of Sport and Physical Education has long recommended 45 minutes a day of gym class for middle and high school students. But most schools fall short of that goal, according to the group. Even limited class time can be wasted since gym is not considered a core subject like math or reading, said Jackie Lund, president of NASPE, an association of fitness educators and professionals.
"There's no national standard, so there's limited accountability," she said.
According to the Shape of the Nation, an annual report by NASPE and the American Heart Association, only 15 states required student assessments in physical education last year.
The Cornell report found that adding an additional 200 minutes of physical education a week resulted in boys spending only about 7 1/2 more minutes being active in gym class.
For girls, an additional 200 minutes of PE resulted in about 8 more minutes of being active in gym each week.
The rest of the extra gym time is likely spent being sedentary -- most likely standing around idly while playing sports like softball or volleyball that don't require constant movement, Cawley said.
"We're not saying schools should get rid of (physical education), but that increasing time alone has no effect. There has to be a meaningful change in the curriculum," Cawley said.
The findings are supported by a past study that showed most gym time in Texas elementary schools was spent being completely sedentary or minimally active. Only about 3 minutes was spent being moderately or vigorously active.
However, Lund says that merely counting how many minutes students are moving may not be a fair measure of a gym class.
"It's not supposed to be aerobics class. The activity level is going to vary depending on the sport they're learning," she said.
Electives like aerobics and yoga can boost physical activity and complement gym class, but Lund said traditional PE class is also critical to develop the cognitive and social skills needed to appreciate sports.
"You're teaching kids habits they're going to practice for a lifetime," she said.
Lund said teachers should aim to have kids moving for at least half of class time.
At its best, gym class can improve the physical fitness of students, said Amy Winterfeld, a health policy researcher at the National Conference of State Legislatures.
"But if it's not a high quality program, then making kids spend more time in them obviously isn't going to help," she said.