SATURDAY, April 7 (HealthDay News) -- Many teens carry school backpacks that exceed 10 percent to 15 percent of their body weight, which puts them at risk for back pain and related disorders, a new study says.

The threat posed by the heavy weight is made greater by the fact that most teens don't get enough exercise, according to the researchers.

The study included more than 1,400 students, aged 12 to 17, in 11 schools in a province in northwestern Spain. The teens were first weighed with the backpack they normally carry to school and then weighed again without the backpack.

The researchers also collected information about the students' height, exercise levels, underlying health problems and back health.

The average weight of the students' backpacks was almost 7 kilograms (15.4 pounds). Nearly 62 percent of the students carried backpacks that exceeded 10 percent of their body weight, and 18 percent carried backpacks that exceeded 15 percent of their body weight.

One in four students said they had experienced back pain for more than 15 days during the previous year. The most common problem was scoliosis, or curvature of the spine, which was diagnosed in 70 percent of the students with back pain. Low back pain and continuous and involuntary muscle contraction were also common problems.

Girls were more likely to have back problems than boys, and their risk seemed to increase with age.

Teens with the heaviest backpacks were 50 percent more likely to have back pain for longer than 15 days, compared to those with the lightest backpacks.

The study appears online in the Archives of Disease in Childhood.

Doctors and teachers need to educate parents and children about the risks of carrying heavy backpacks to school every day, the researchers said.

More information

The American Academy of Orthopaedic Surgeons has more about backpack safety.