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AIROBI, 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.

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.