No Skycaps Needed

By Carl Zimmer|Tuesday, August 01, 1995
One of the most startling sights on a first trip to Africa is a common one: women carrying things on their heads. Try to carry a suitcase on your head, and you’ll probably bite your tongue in concentration and wave your arms madly for balance. But African women walk for miles with heavy jugs of water or pots of food as if they weren’t carrying anything. Energetically speaking, they aren’t: researchers have found that the women can carry enormous loads without using any extra energy. They aren’t defying any laws of physics, though; they’re being good pendulums.

In 1977 a team of Harvard physiologists, in Kenya to study the locomotion of wild animals, found themselves distracted by the load- carrying women. When we tried to pick up the loads, they were just amazing, says Norman Heglund. We wondered how the devil they did it. As a first experiment, Heglund’s team convinced some Kenyan women to wear breathing masks as they carried their loads; the idea was to measure the women’s oxygen consumption, calculate how many calories they were burning, and then compare their performance with that of non-Africans. There the experiment ran into a snag: the non-African researchers couldn’t match the carrying capacity of the Kenyan women, at least not with their heads. Heglund and his colleagues had to resort to backpacks and to using old measurements from American Army recruits.

Still, the results were extraordinary. The African women could carry a fifth of their weight without burning a single extra calorie; and although larger loads did require more energy, the increase was only half of that needed by the American soldiers. Some women could carry 70 percent of their weight.

Funding agencies haven’t exactly been desperate for the answer to this riddle, so it’s only recently that Heglund has managed to get a step closer to one. While spending a year teaching at the University of Nairobi in 1989, he had some Kenyan women walk across force plates; last year in Belgium he repeated the experiment with European students. Force plates are devices that register the vertical and horizontal forces exerted by a walking animal.

A walking human is like a pendulum swinging. When the pendulum is at its lowest point, it is moving fastest, and its energy is almost all kinetic energy of motion. As the pendulum climbs up one side of its arc and is slowed and finally stopped by gravity, that energy isn’t all lost; most of it is stored as potential energy and is converted back into kinetic energy when the pendulum starts to fall again. But some of the energy is lost to friction, both in the bearing and between the pendulum and the air.

Similarly, when you walk, the kinetic energy of your forward movement turns into potential energy as you rise on one foot and is converted back into kinetic energy as you fall onto the other foot. But with each footfall, only 65 percent of that kinetic energy is carried over into the next step; 35 percent is lost, mostly to internal friction in your leg. That 35 percent has to be made up by your leg muscles, which convert food energy into kinetic energy.

The 35 percent rule applies to Kenyan women too--until they start carrying things on their heads. Heglund’s force plate readings allowed him to calculate how much energy his subjects were transferring from one step to the next. Without a load, Kenyan women and Europeans both transferred 65 percent. When the Europeans carried loads on their backs, they still lost 35 percent--but now, since they were bearing more weight at the same speed, that 35 percent represented more energy in absolute terms, which they made up by burning more calories. In contrast, the Africans simply became better pendulums. When they carried a fifth of their body weight on their heads, they somehow managed to transfer 75 percent of their energy from one step to the next, losing only 25 percent to friction. With a greater load, one woman reduced her loss to 15 percent.

Heglund doesn’t know what biomechanical trick the Kenyan women are using--they couldn’t tell him--but it must have something to do with carrying things on your head. People who carry things for a living, he notes, from Kenyans to Sherpas, tend to use their heads. It’s just amateurs like us that use suitcases and backpacks, Heglund says.
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