Lugging more than 10 percent of his body weight (which isn't much -- about five and a half pounds for a 55-pound child) can be a serious pain in a kid's back. Some ways to lighten the load:
Choose a backpack that has two wide straps with good padding to distribute the load evenly.
Adjust the height. "Kids like to leave the straps loose so the bag hangs off their back," says Timothy Neuschwander, M.D., lead author of a study conducted at the University of California, San Diego, on back-pack side effects. "That may cause pain because the lower back is carrying all the weight, rather than the shoulders doing more of the work." Tighten straps so the bag is centered on the back, ideally above the lower curve of the spine.
Use both straps. The one-shoulder method may be cool, but it adds serious pressure and throws body alignment out of whack.
Weigh the bag. Plunk it on a scale, then have your kid step on (without the backpack). Do the math, and pull things out of the bag until you get below that 10-percent-of-body-weight limit. If he regularly has to carry more than he should, get him one of those rolling backpacks that he can pull.
Have him checked. If your kid complains of back pain-even if it seems minor-talk to your doc. No amount of achiness is normal, and a physician can recommend strengthening exercises to help ease it.
Plus: Check out Parenting.com's 30+ Best Backpacks for Kids