High school girls are more likely to exercise vigorously if they're in girls-only physical-education classes that offer a variety of activities such as dancing, aerobics and brisk walking, a new study reports.

And they do better if the school in general promotes activities for girls, says the report in September's American Journal of Public Health.

Public health officials are concerned by other research showing that girls become much less physically active during their teen years, and many don't exercise at all by the time they reach 18 or 19. Government guidelines recommend that children be active at least an hour a day.

Russell Pate, professor of exercise science at the University of South Carolina, says girls long have been given exercise classes designed more for boys, and "a lot of them don't like it very much."

Efforts over the past 30 years to level the physical-education playing field may have backfired.

During the 1950s, 1960s and early 1970s, girls and boys usually were separated in junior high and high school programs, but "after Title IX was put into place by the mid-'70s, coed physical education became the more common approach," said Judy Young of the American Alliance for Health, Physical Education, Recreation and Dance, a professional organization that includes teachers, researchers and administrators.

"The concern then was that girls weren't having full access to the same resources in terms of content, facilities and equipment. Girls were being shut out of activities like weight training and wrestling that were commonly offered to boys."

Raymond Fujino, a civil rights compliance specialist for the Hawai'i Department of Education, said the law prevents segregating classes by gender, except in the case of contact sports such as football and wrestling. "You can't separate them in a P.E. class right off the bat," he said.

While former U.S. Education Secretary Rod Paige introduced an initiative that would allow for all-boys and all-girls gym classes, that initiative is still under discussion, Fujino said. Unless that passes, Hawai'i schools will be required to offer coed gym classes, although boys and girls may be separated for specific units.

Experts speculate that schools might not be attuned to the kinds of exercise their female students really enjoy.

Pate and his colleagues chose 24 high schools associated with 31 middle schools in South Carolina. The researchers measured the height, weight and activity level of 2,744 girls in eighth grade and again in ninth grade.

Ninth-grade girls at 12 of the high schools took traditional coed physical ed classes.

At the other 12 schools, the girls participated in such activities as aerobics, dance, exercise walking, self-defense and strength training in addition to organized sports. In most cases, these classes were girls-only, which many girls like because they are self-conscious around boys, Pate said.

Also, at the nontraditional schools, photos of girls doing activities were posted on bulletin boards.

When activity levels were checked at the end of ninth grade, the researchers found that almost half of the girls taking nontraditional gym averaged one or more 30-minute periods of vigorous activity a day over three days. Those levels compared with a little more than a third of girls exercising that much in the traditional programs.

One size doesn't fit all when it comes to activity for girls, according to Pate.

"Girls are very diverse in what they enjoy. Athletic girls are comfortable with vigorous physical activity and competitive sports, but a lot of girls much prefer less intense activities," he said.