Taking burden in their stride
(appeared in Nov 2016)

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The Nepali porter is found to walk lighter when the load gets heavier, says S.Ananthanarayanan.

The African tribal and the eastern coolie seem to be the leaders in carrying great loads and over long distances. As for heavy loads, nobody who has been on a Himalayan trek can help being impressed by the sheer weight that the diminutive Sherpa porter cheerfully hauls up and down the mountains.

G. J. Bastien, P. A. Willems, B. Schepens and N. C. Heglund, from Institute of Neuroscience, Laboratoire de physiologie et biome´canique de la locomotion, Université Catholique de Louvain, in Belgium report an in depth study of how much energy is consumed, and how usefully, in the way that westerners, African women and the Nepali porter transport loads. The report appears in the Journal of Experimental Biology, which is published by The Company of Biologists, a 90-year-old not-for-profit publishing organization dedicated to supporting and inspiring research in the life sciences.

Following up on other, earlier research in the field, the Louvain group notes that not many studies on the actual energy efficiency and biomechanics of people carrying loads have been published. The group has thus gone into just that, with the help of published studies of energy consumption and experiments on the actual forces at work in the gait of walkers, and analyses of energy efficiency at different speeds and under different loads.

The study cites earlier findings that fit European or North American adults can carry a backpack up to a quarter of their weight over a day’s trek. If the load is more than 60% of their weight, however, they last only an hour and it is only with great difficulty that they can carry as much as their own weight. In other parts of the world, on the other hand, women have adapted to carry heavy loads efficiently and seemingly much frailer men, even boys, routinely carry well over 100% of body weight over long distances and for long periods. The group hence studied details of the known efficient technique of the African women and whether this was made use of by Nepali porters, who are noted for carrying heavy loads.

Kenyan women are renowned for carrying as much as 70% of their weight, with apparently little exertion. Earlier studies have shown that this is thanks to their technique of converting the energy of the up and down motion of the total load, of their body and the burden, into power to propel them forward and again to use their forward motion to help raise the load for each step. This is explained as similar to a pendulum, where the bob gains speed as it falls to the lowest point of its swing, and then makes use of the speed to rise till the opposite end of its swing. The Kenyan woman, in turn, acts like a pendulum stood upside down, speeding forward as the load descends when the carrier steps forward and using the same speed to help raise the load, when the weight transfers to the other leg. The earlier study, by Heglund and others found that Kenyan women could carry a load up to 20% of their body weight without any additional effort. The study has shown that when she carries no load, the Kenyan woman walks in the same way as westerners. With a load, however, her efficiency of energy use rises from 65% to 80%, which means she uses only 20% extra energy to keep moving, unlike westerners who need to use 35% more energy. Walking with a heavy load on the head, of course, calls for balance and rhythm, but the result is very high conversion of energy expended into useful work done!

Nepali porter

The lower Himalayas and Nepal are areas of sparse produce and rough and undulating terrain. At there are few roads, there is the need to transport materials over distances, as head-loads. And, as the foot paths are rocky and uneven, the speed of elegant movement in the plains is not possible. For all this, professional Nepali porters, who carry the load with the help of a strap looped over the forehead, similar to Kenyan women, do a 100 km trip in some eight days, with total ascent of more than 8,000 metres and descents of over 6,300 metres, and carrying a load of 89% of body weight, on the average, with 20% of the men carrying over 125% of the body weight, the study says. The team hence went into the details of the gait and the energy use by westerners, the African women and the Nepali porters.

The actual work done, in the sense of the up and down and other movements of the body and the load, was exactly assessed, by filming the movements of the subjects – the Nepali men, Kenyan women and westerners, as they walked, and then analyzing the footage. While this showed the external work done, for the motion of the body mass and the load, other forms of work done were assessed with the help of five force sensor plates in a force platform over which the subjects walked. Each plate had four sensors to measure forward, backward and vertical forces and the measurements were digitized and recorded twenty times every second. These figures, obtained with the different classes of subjects and at different speeds and under different loads, thus measured the work done. This measurement was combined with available data, for the same conditions of speed and load and category of subjects, of the metabolic cost of the work, to work out the efficiency of load carrying.

The result of the detailed study is that the Nepali porter does not use the energy saving methods of the Kenyan woman, and indeed, he cannot, as the terrain he moves over is hilly and he ‘barely ever takes two steps in succession at the same level’. The Nepali porter’s muscular efficiency is about the same as that of the westerner, and does not depend upon the load, but it decreases if the speed is more than 1.4 metres per second. That efficiency does is not related to the load is in contrast to the westerner, whose muscular efficiency falls fast if the load exceeds 35% of the body weight. The Nepali thus has a smaller metabolic cost in carrying the load over a given distance.

The study says that the Nepali is the most efficient when walking slowly and with a heavy load. When they are behind time, and need to reach a market, for instance, the study says, Nepalis would prefer to walk long hours, maybe late into the night, but they would not step up their speed. Their lower metabolic cost, the study says, seems to arise from having adapted to make less muscular contractions and also their training and adaptation to the higher altitude.

Asian coolie

Adaptation to efficient load carrying is also seen in most parts of South East Asia, as there has been need to transport materials by head-load. Unlike only a pendulum-like gait, as in Africa, the Asian also uses mechanical devices to minimize the up-down movement of the centre of mass. This helps the effort made to be mainly for forward acceleration and less for supporting the load.

The device used by water carriers and porters is to suspend the load at the two ends of an elastic beam supported by the shoulders or on the head. When the carrier takes a step, the load does not rise as high as the shoulders rise, because of the elasticity of the beam. Again, when the carrier descends, there is insulation of the plunge of the load, which is also less as it did not rise to the full extent in the first place. The dimensions of the beam and the speed of walking are matched so that energy is stored in the bent beam and released just when needed. Hitting the right rhythm can lead to the head-load making very little actual up-down movement while it speeds along.

The journal, Nature once carried an account of an ergonomic backpack, which was suspended by elastic straps, to mimic the retarded movements of the Asian porter’s load. The arrangement was said to reduce the metabolic cost of moving with a 27kg load by about 7%.


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