Complete article at:
http://www.nytimes.com/2002/03/12/health/12HEAD.html
Excerpts:
NAIROBI, Kenya — As dawn breaks, Linnette Otieno leaves her small house on Nairobi's outskirts and walks five miles to market. On her head is a load of firewood she plans to sell. The load weighs about 65 pounds. She hardly sweats.
"I've been doing this since I was 6," she explains as she hoists the wood onto her head with an experienced motion.
When she was growing up in her home village in western Kenya, she had to walk even further to gather firewood, up to eight hours a day. By now, at age 35, she says long journeys with heavy loads are second nature.
Scientists have long wondered how women like Ms. Otieno are able to carry so much so easily. Now, in a study to be published shortly, two researchers from Europe describe the trick in detail: women from certain African tribes unconsciously modify their gait to walk using less energy. The energy they save is applied to carrying the weight.
They found that the women could carry 20 percent of their own body weight with no additional exertion. "There wasn't even a blip in their oxygen usage," Dr. Heglund said
In normal walking humans, only 65 percent of their energy is actually transferred from one step to the next; the rest is dissipated and must be replaced by additional muscle energy.
But the African women have a secret weapon, the researchers discovered. As they transfer their weight, they transfer at least 80 percent of their forward energy to the next step. Only 20 percent must be replaced by the muscles, leaving plenty of energy in reserve to carry the weight on their heads.
At the height of each step, the normal walking human begins to drop down, losing potential energy without transferring it into kinetic energy, which would generate additional forward speed. The African women, however, are able to minimize this loss through a tiny alteration of their gait.
It's a tiny difference that is almost invisible to the naked eye, and "even the women don't know how they do it," Dr. Heglund said. But with a sophisticated training program, he went on, "you could train other people to do the same thing."
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