Schoolwork can be a pain—especially when it's borne on a child's back. There have been a lot of worries about spine problems—never proved—but parents may be surprised to learn about another possible form of harm: Those bulging backpacks might actually be cutting off blood flow.

In this week's Archives of Pediatric and Adolescent Medicine, a research team based at the University of California–San Diego reported that after they loaded 10 kids with backpacks weighing about 27 pounds—or 20 percent of their body weight—the pressure on their shoulders far exceeded that known to compress blood vessels. That could spell big trouble because plenty of kids carry even heavier packs; some reports put the typical load at 22 percent of body weight.

Blocked blood flow can cause muscle pain, though one of the scientists, Gita Murthy of the environmental consulting firm Rore Inc., admits that this research didn't actually show blocked flow. What it did show was pressure strong enough to cause a block, particularly on the shoulders. And she says kids make it tougher on themselves by carrying the bags "very low, way below their butts, so the straps only contact a small area in front of the shoulder." Better to wear them higher, and get models with wide straps to distribute the load across the shoulders.

This discovery of a shoulder-pain mechanism fits with other research showing that the shoulder is more likely to get hurt by backpacks than is the back itself. The shoulder is the fourth-most-common injury site, while the back ranks sixth, according to research published in 2003 in the journal Pediatrics, based on injuries reported to 100 emergency departments throughout the United States. The body part hurt most frequently, by the way, is the head. Why? Because most backpack injuries happened not when kids were wearing the heavy bags but when they tripped over them.