African women who carry enormous loads on their heads -- sometimes up to 70 percent of their body weight -- expend much less energy per pound of load than do soldiers carrying backpacks or than even pack animals.

In fact, according to a Kenyan, Italian and American team that published its findings in this week's edition of the British journal Nature, these women could carry up to 20 percent of their body weight without expending more energy than they use to walk without a load. Soldiers had to raise their energy output by 13 percent to carry a 20 percent load.

When carrying a 70 percent load the women raised their energy consumption only about 50 percent. The soldiers raised theirs by 100 percent.

The findings, which the scientists said they could not explain for certain, were obtained by putting Kenyan women on treadmills and measuring their rate of oxygen consumption. The body gets energy by using oxygen to "burn" sugar. Hence, the amount of oxygen inhaled but not exhaled is a measure of the amount of energy being expended.

The women tested were from the Luo tribe, which traditionally carries the burden atop the head, and from the Kikuyu tribe, which carries it behind in a sling hanging from a strap across the forehead. Energy efficiencies for both methods were the same.

One speculation as to how the women do it suggests that head-supported loads are moved less up-and-down or side-to-side with each step than are back- or shoulder-supported loads. These tiny motions, along with a repeated slowing and accelerating at each step, probably cost extra energy. Experience also seems to be a factor. People unaccustomed to head-carrying use up as much energy that way as carrying the same load on the back.

The researchers speculate that African women, who often have carried loads on the head daily since childhood, may develop anatomical adaptations that smooth the load's movement.

The research was done by G.M.O. Maloiy of the University of Nairobi; N.C. Heglund, L.M. Prager and C.R. Taylor of Harvard University and G.A. Cavagna of the University of Milan.