Académie royale de Médecine de Belgique

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Vidéo et résumé de Norman Heglund

 

(Séance du 23 mars 2013)

THE ENERGETICS AND MECHANICS OF CARRYING HEAD-SUPPORTED LOADS BY AFRICAN WOMEN

par N. HEGLUND (UCL), invité.          

When travelling in East Africa one is often surprised at the prodigious loads carried by the women of the area. It is not uncommon to see women of the Luo tribe carrying loads equivalent to 70% of their body mass balanced on the top of their heads. Women of the Kikuyu tribe carry equally large loads supported by a strap across their foreheads. We found that both the Luo and Kikuyu women could carry loads of up to 20% of their body weight without any increase in their rate of energy consumption. For heavier loads there was a proportional increase in energy consumption, e.g. a 30% load increased the energy consumption by 10%, a 40% load by 20% and so on. For comparison, soldiers carrying loads in backpacks have been extensively studied, and they show, for example, a higher rate of energy consumption with a 70% load at all except at the very lowest walking speeds. At the optimal walking speed, this load would double a soldier’s metabolic rate, but would only increase the African women’s metabolic rate by 50%. Unlike the African women, the soldiers were unable to carry any load for ‘free’.

In an effort to explain how the African women are able to carry loads so economically, we investigated the biomechanics of carrying head-supported loads using force platforms. We found the weight-specific mechanical work required to maintain the motion of the common centre of mass of the body and load decreased with load in the African women, whereas it increased with the control subjects. This decrease in work by the African women is a result of a greater conservation of mechanical energy resulting from an improved pendulum-like transfer of energy during each step, in other words, a better exchange between the gravitational potential energy and the kinetic energy of the centre of mass.

References :

Cavagna, G.A., P.A. Willems, M.A. Legrammandi & N.C. Heglund. 2002. Pendular energy transduction within the step in human walking. J. Exp. Biol. 205:3413-3422.

Cavagna, G.A., P.A. Willems & N.C. Heglund. 2000. The role of gravity in human walking: pendular energy exchange, external work and optimal speed. J. Physiol. (Lond.) 528: 657-668.

Heglund, N.C., P.A. Willems, M. Penta & G.A. Cavagna. 1995. Energy-saving gait mechanics with head-supported loads. Nature 375:52-54.

Maloiy, G.M.O., N.C. Heglund, L.M. Prager, G.A. Cavagna and C.R. Taylor. 1986. The energetic cost of carrying loads: have African women discovered an economic way? Nature 319:668-669.

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Ont pris part à la discussion : MM. les Prs J.-B. Otte, B. Van den Eynde, P. Vanderhoeft, Mme K. Udovenko (invitée), Th. Godfraind, Ph. Lefèbvre et S. Van Sint Jan.