Children get less exercise as they get older. That is the finding of a years-long study tracking the activity of 1,000 American children.

The average amount of daily physical activity dropped from about three hours a day for 9-year-olds to less than an hour during the week and even down to 35 minutes on weekends for 15-year-olds. By that age, according to the study by the American Medical Association, a little more than a quarter of the older children met the government's minimum recommended one hour of moderate to vigorous exercise every day.

The study found that physical activity dropped sharply for girls at 13 and boys 141/2 years old. Boys tended to be more active than girls at the four age levels measured. The study measured activity from walking to bicycling and sports using monitoring devices worn by the children in 10 cities for a week at ages 9, 11, 12 and 15.

The drop in activity concerned physicians troubled by the increase in childhood obesity along with heart disease and other illnesses associated with it in youngsters and adults.

"People don't recognize this as the crisis that it is," said Dr. Philip Nader, a pediatrician and professor emeritus at the University of California at San Diego who was the lead author.

The decline in activity has been often noted, especially as children turn to less physical activities such as video or computer games and use of the Internet. Dr. Nader also cited a decrease in physical education classes in school. Children also lose opportunities to play organized sports as they grow older as the sports become more competitive.

The report is the second in two weeks to raise the alarm about childhood diseases. The American Academy of Pediatrics sounded a warning about rising cholesterol levels in children as young as 8 and recommended that children in some cases be given medication to control their cholesterol to avoid possible heart disease and other medical problems associated with an increase in childhood obesity. Nearly one-third of today's children are considered overweight.

The AMA study, though, suggests that medicine may not be enough. The AMA study reinforces the need to find alternatives, to encourage children and to teach them lifelong habits such as the value and necessity of exercise at every age to maintain healthy lifestyles.