Yaël: Hi, I'm Yaël Ksander, we've got a great show for you today, right Don? Don?

Don: [PAINED] Over here...

Y: What are you doing lying on the floor?

D: All I did was walk over to the studio with my backpack, and now my back is killing      me!

Y: Hmm. Let me see that backpack. This thing weighs a ton! Experts say that overly      heavy backpacks can cause back pain. That's true especially for kids. Did you know      that something like twenty-percent of kids missed school last year because of back      pain caused by backpacks?

D: But I thought backpacks distributed weight across the back in a way that makes it      safe or something.

Y: The spine has thirty-three vertebrae, right? And between the vertebrae are discs that      cushion the bones and allow the spine to bend. Putting too much pressure on the      discs, though, can cause pain. That's the problem with a really heavy backpack. A      pack that's more than around fifteen-percent of a kid's body weight will pull her      backward. Leaning forward to compensate or arching her back compresses the      spine and really squeezes those discs, which can cause pain in the neck, back and      shoulders. Also, wearing a backpack over one shoulder can hurt because it pulls the      body to one side. Compensating by leaning to the other side puts a lot of pressure on      the spine, too.

D: Wow. Are the injuries permanent?

Y: Usually not. But it still hurts enough to keep kids out of school.  

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A Moment of Science^®