It’s documented that children shouldn’t carry more than 10 to 15 percent of their total weight in their backpack. Researchers in Spain assessed the backpacks and back health of 1,403 students ages 12 to 17. They concluded that over 60 percent were carrying backpacks weighing more than 10 percent of their body weight. One in four reported suffering back pain for more than 15 days during the previous year. Other conditions included stress fractures in the back and nerve damage in the neck and shoulders.