Not too long ago, the most complicated educational tool a kid had to use was a retractable ballpoint pen. Then came the computer, and everything changed, including the way kids work, play, and sit. And as the computer flourished, so did things like muscle strains and repetitive stress injuries.

Fortunately, ergonomics -- the science of designing and arranging equipment to promote comfort and avoid injury -- flourished as well. To help you equip your family for everyday life in the modern world, we asked ergonomics experts and savvy parents to share their tips for comfortable computing, backpack fitting, and more.

MAKE COMPUTING COMFORTABLE

Just as you wouldn't buy your child adult-size sneakers, you shouldn't expect him to sit at a computer station designed for adults. If your family shares a computer, a few small changes will go a long way toward increasing everyone's comfort.

"If I had to make one proclamation, I'd say put your money into an adjustable chair that everyone in the family can use," says Cheryl Bennett, organizer of the Ergonomics for Children and Educational Environments Technical Committee. Whatever equipment your family has, it's essential that you position it correctly and that it suits everyone who uses it.(See The Family Workstation for specific tips.)

The keyboard, for example, should be under your fingers when your elbows are bent at a 90-degree angle. An adjustable-height keyboard tray can help your child here, or, like Pamela Adams, a San Francisco chiropractor and mother of three, you can set your keyboard and mouse on a large serving tray that your child can place on his lap.

Also helpful are kid-size keyboards, which are designed expressly for little fingers. If your child has his own computer, consider investing in an adjustable desk that can grow with him (the Kidstation Ergonomic Single Computer Desk and Chair, for example, runs about $200). And, to help reduce eye fatigue, position your computer away from bright sources of light such as a window, and be sure there aren't lights reflecting on the screen.

HAVE A SEAT

Sitting properly at the computer is also key, but if your child resists your entreaties to sit up straight, don't fret. Notes Alan Hedge, an ergonomics professor at Cornell University, "What we think of as proper posture these days is a more relaxed position: sitting back in a chair in a comfortable way, with all body parts supported and nothing outstretched or bent away from the body."

Kids' natural tendency to fidget actually works in their favor. "It's vital to move," says Cheryl Bennett, who uses a timer to remind her daughter to take a 2-minute walking or stretching break every 20 to 30 minutes (the amount of time most bodies can remain comfortable in one position). Patrice Winter, spokesperson for the American Physical Therapy Association, takes advantage of the fidget factor by having her kids sit on exercise balls, "which require balance and work the core muscles, the deep muscles around the spine."