When Dr. Rick Akau and his wife, Sabrina Toma, of Honolulu, took their daughters, Kalei, 8, and Ke'ala, 5, shopping for back-to-school backpacks, they weren't looking for the latest fad; they were concerned with form and function.

Backpacks are more than a fashion statement. They are an efficient way to carry a heavy load and can be beneficial if used properly.

Backpacks are preferred over shoulder bags because the back and abdominal muscles, among the strongest in the body, support the weight of the backpack. When worn properly, a backpack can also distribute the weight evenly across the body, so neck and shoulder injuries are less common.

Akau, a physiatrist (a medical doctor specializing in care and prevention of musculo-skeletal conditions) with Straub Clinic&Hospital's Bone and Joint Center, has seen too many neck, back, wrist and shoulder problems caused by carrying or pulling too-heavy loads. "Pinched nerves, numbness in the arm, weakness in shoulder muscles — I've seen it all," Akau said.

Akau has also heard of traumas in emergency rooms and pediatricians' offices caused by backpacks being tripped over in school hallways or a child whacking another child with the pack while trying to pass through a narrow hallway or bus aisle.

Akau is not about to let his daughters suffer from backpack abuse.

The Akaus first familiarized themselves with the campus areas their daughters would need to navigate. They reviewed locker locations, stairs, tree roots and other potential obstacles.

They determined that Kalei, who is entering the 4th grade at Punahou School, needs a rolling backpack because she has to carry so many books, in addition to her lunch, a plethora of supplies and a change of clothing for her after-school dance classes. Since the Winne units, where her classes are held, are fairly level and do not have stairs, she will not need to lift up her backpack.

Ke'ala, however, faces a flight of stairs at the entrance to her kindergarten classroom in Wilcox Hall, so she needs a backpack she can carry. The Akaus looked for a lightweight pack that would accommodate not only her school supplies but a sleeping mat, change of clothes and jacket.

School backpacks come in various sizes appropriate to different ages. The bottom of the pack should rest in the curve of the lower back. It should never rest more than four inches below the child's waistline, according to the American Occupational Therapy Association.

LOOK FOR THE LIGHTEST

Since physicians recommend that a student carry no more than 10 percent to 15 percent of his or her total body weight, the Akaus wanted to find the lightest possible backpacks for their daughters. They had the girls test all the zippers and try to imagine all their "stuff" in the various pockets and sections. Naturally, aesthetics played a role, but not until the packs were determined to be practical.

Ke'ala chose a lavender pack from K-Mart ($12) with outside pockets for the supplies she needs immediately.

Kalei found a pretty floral rolling backpack at Gap ($25 on sale) that her father praised for its lightweight but sturdy plastic wheels.

Laurel Bowers Husain, a Punahou parent and staffer, recommends students put laptop computers in a separate well-padded bag. "They shouldn't be put in backpacks because the kids really throw around their backpacks" and that could be the death of a laptop, she warned.

She advises students to use lockers as a base for heavy objects: "Just take the books you need for each class with you and leave the rest in your locker."

Akau said he plans to check his girls' backpacks weekly to make sure they aren't carrying around a lot of extraneous stuff. He discovered that Kalei had two bottles of water in hers, so he suggested she take one out. Paper is particularly heavy, so books, notebooks and folders that aren't needed for the day should be kept elsewhere.

Since teens are often protective of their privacy, the backpack can be viewed as a particularly private thing. "In the sensitive teen years, it might be best to simply encourage your student to clean out his or her own backpack," Akau advised.

At Office Max on Ala Moana Boulevard, manager Cynthia Dalberg said the best-selling backpacks this year are those with wheels. She recommends packs with lots of pockets for special items such as cell phones and water bottles, as well as a clearly visible slot where a card with emergency numbers can be seen.

Sean Hatada, a sales associate in the backpack section of Sports Authority on Ward Avenue, said backpacks with wheels are generally the best-sellers at that store, too. However, the single most popular style is a JanSport pack with a rawhide bottom, "because it's so durable."

The Sports Authority customer tends to prefer black and navy, but floral prints are selling well this year. Pink, however, is not.

PACK IT LIGHT, WEAR IT RIGHT

The American Occupational Therapy Association has established a nationwide initiative called Lighten Up! to encourage safe backpack strategies for parents and students. They advise that both shoulder straps always be worn. Wearing a pack slung over one shoulder can cause a child to lean to one side, curving the spine and causing pain or discomfort.

The organization also recommends choosing a pack with well-padded shoulder straps, because shoulders and necks have blood vessels and nerves that can cause pain and tingling in the neck, arms and hands when too much pressure is applied.

Parents should adjust the shoulder straps so the pack fits snugly to the child's back. A pack that hangs away from the back can strain the neck and back muscles.

Wear the waist belt if the backpack has one, as this helps distribute the weight more evenly.

For more information, go to www.aap.org and enter "backpack" or see www.aota.org/backpack/links/schifact.asp.

Of course, the days of textbooks and homework on CD-ROMs may not be far away. That will be a boon to the backs of students everywhere.