A new study of high-school students has found that only 37 percent say they engage in regular exercise, while another 35 percent say they watch television for at least three hours after school each day.
Male students were more likely to be vigorously active than females: 50 percent of boys said they exercised regularly, against 25 percent of girls. And the girls' activity levels decreased as they reached their senior year of high school, according to the survey, published in the current issue of the Archives of Pediatrics and Adolescent Medicine.
Risk factors linked to heart and blood-vessel disease are known to establish themselves during childhood and adolescence, according to Gregory W. Heath of the Centers for Disease Control and Prevention in Atlanta, the study's lead author.
"During adolescence, many of these same risk factors, including overweight and obesity, increased levels of blood pressure, increased levels of cholesterol and increased anxiety and depression, are linked to physical inactivity," Mr. Heath wrote.
Several studies have suggested that physical-activity levels during childhood and adolescence predict a person's degree of activity as an adult, he added.
In the new study, African-American girls had the lowest activity levels, and 60 percent of them said they spent three or more hours each day after school watching television.
"These patterns are likely to contribute to the pattern of inactivity seen among black women, and may lead to the excess of obesity, hypertension, diabetes and cardiovascular disease among black women," Mr. Heath wrote.
The researchers studied responses to the 1990 Youth Risk Behavior Survey, which was designed to measure the prevalence of health-risk behaviors among youth. The survey was distributed to schools in all 50 states and the District of Columbia, and was completed by 11,631 students in 124 schools.