Photographer's Note

After seeing Gert's photo of the Miss Roadworker I looked in my a bit old archive. I often wonder how the women in exotic countries can go with the heavy load on their heads. They use something under the basket or basin (the first woman has it in her hand), but still it is often heavy. In a Jearl Walker book "Flying Circus of Physics" I found an explanation. I doubt that you will read it , and I doubt even more that you will understand it (I haven't, although I was editor)) but here it is anyway.

In some cultures, such as in Kenya, women can carry enormous loads on their heads. They may have strong neck muscles and an acute sense of balance, but the really surprising feature is how little effort is required of them. For example, a woman might be able to carry a load up to 20% of her body weight without having to breathe heavily (in fact, without any extra effort on her part), whereas a European or American woman of comparable health and strength would find carrying such a load very difficult. What is the secret of the elite load bearers?

During walking, a person’s center of mass moves
up and down in a periodic fashion. The high point occurs when the body is over one foot while the other foot is moving past that foot, toward the front. The low point occurs when both feet are on the ground and her weight is being shifted from the rear foot to the front foot. This periodic vertical
motion of the center of mass, with the support point
periodically moving horizontally beneath the center of mass, is similar to the motion of a unicyclist who moves back and forth to maintain balance. In particular, part of the woman’s energy is periodically shifted between potential energy (related to the height of her center of mass) and kinetic energy (the speed at which her center of mass moves forward) .Normally a person is inefficient in the energy transfer for about 15 milliseconds just after the high point is reached. That is, as the center of mass begins to descend, not all of the potential energy is transferred to kinetic energy, and muscles are used to propel the person forward.
An elite load bearer walks in this normal and slightly inefficient way when she is not carrying a load. However, when she carries a load, the interval of inefficiency just after the high point is reached is less. In fact, carrying a moderate load (20% of body weight) may require no more effort than carrying no load at all, presumably because the load
causes the woman to shift potential energy to kinetic energy more efficiently than normally. Only if the load exceeds 20% of body weight does a woman have to expend more energy than when unloaded, but even then she expends less energy than, say, a European woman who walks differently.

I usually go with my backpack everywhere and I feel strange without it. Very often it is heavy as 20% of my body weight when I make shopping.

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Additional Photos by Malgorzata Kopczynska (emka) Gold Star Critiquer/Gold Star Workshop Editor/Gold Note Writer [C: 11629 W: 123 N: 29558] (139176)
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