Physiology of load carrying
East African Kikuyu and Luo women, farmers in the Andes, and the Sherpa in Nepal are just a few examples of people who must carry large loads everyday, and who choose to carry the loads supported by their heads. In fact, world-wide (with the exception of Western Europe and North America) people who must carry heavy loads often choose to carry the loads on their heads, apparently for good reason. Our previous studies on the East African women have shown that they can carry loads of up to 20% of their body weight (equivalent to a good size suitcase) for 'free', i.e. for no increase in their metabolism. Furthermore, these women can carry up to 70% of their body weight on their heads considerably more economically than their European counterparts can carry equivalent backpack loads. For example, an African woman can carry a load equal to 70% of her body weight at 3.5 km/hr for 50% less energy that an American army recruit with the same load in a backpack. Studies from other laboratories on West African and South African women, when adjusted for obesity, are substantially in agreement.
We have recently been able to show how these women are seemingly able to ‘defy the laws of thermodynamics’ and ‘get something for nothing’. The explanation is wrapped-up in the mechanics of the walking gait. When a person (or animal) walks, their body goes up and down, and goes faster and slower, within each step. The energy changes associated with these fluctuations in height and speed are out of phase and therefore tend to cancel each other, minimizing the energy required to keep the movements going, much like in a pendulum. But in walking the energy fluctuations are not completely cancelled (as would occur in a perfect pendulum); at most about 65% of the energy fluctuations are cancelled, leaving at least 35% of the energy fluctuations which must be supported by the muscles each step, requiring metabolic energy input.
When the African women carry loads on their heads, they are able to increase the amount of energy that is cancelled, reducing the muscular energy required to maintain the walking gait and compensating for any increase in muscular energy required to support the additional load. In contrast, the European control subjects carrying backpacks are unable to increase the cancellation in their energy fluctuations, and their metabolism increases in proportion to the load.
The question remains, are these African women unique in their ability to carry moderate loads for free and large loads cheaply? When we tried to carry loads on our heads, we were unable to save any energy, and we only managed to get a stiff neck in the process. The only other studies on the energetics of carrying head-supported loads were done in India. Two studies, one using untrained office workers and the other using professional porters, show the cost to be about 50% greater than African women carrying the same loads at the same speed; the third study, again done on professional porters, paradoxically shows the cost to be about 30% less than the African women.
In this project, we studied the most famous head-supported load carrying people, the Sherpa of Nepal. These porters have been known to carry, in an emergency, considerably more than their own body weight on their heads. More typically the men carry 50 kg loads and the women carry 40 kg loads, or 80 and 60 kg loads respectively for short distances. We plan to make the following measurements:
- Oxygen consumption rate (equivalent to metabolic rate when done in a steady state condition) will be measured as a function of speed using the new portable K4 system, both unloaded and loaded with a single moderate-size load. This is to determine the speed at which the minimum oxygen consumption rate occurs.
- Oxygen consumption rate at the speed where the minimum was found as a function of load. This is to determine the relation between cost and load for the Sherpa.
- The mechanical work done when walking with head supported loads will be measured as a function of speed and load using a force platform system and infrared cameras. These measurements will allow the calculation of the energy transfer, the resulting energy cancellation, and the efficiency of carrying head supported loads.
More information: Norman Heglund
Publications of the lab on load carrying
BASTIEN G.J., SCHEPENS B., WILLEMS P.A. & HEGLUND N.C. (2005) Energetics of load carrying in Nepalese porters. Science, 308: 1755
BASTIEN G.J., WILLEMS P.A., SCHEPENS B. & HEGLUND N.C. (2005) Effect of load and speed on the energetic cost of human walking. Eur J Appl. Physiol., 94: 76-83
CAVAGNA G.A., WILLEMS P.A., LEGRAMANDI M.A. & HEGLUND N.C. (2002) Pendular energy transduction within the step in human walking. J. exp. Biol. 205: 3413-3422
BASNYAT B. &SCHEPENS B. The burden of the Himalayan porter (2001) High altitude medecine & biology 2: 315-316
BASTIEN G., WILLEMS P.A., SCHEPENS B. & HEGLUND N.C. (2001) No free load for porters in Nepal Arch. Physiol. and Biochem, 109: S121
SCHEPENS B., BASNYAT B., WILLEMS P.A. & HEGLUND N.C. (2001) Measurement of the loads carried by porters in Nepal Arch. Physiol. and Biochem, 109: S72
HEGLUND N.C., WILLEMS P.A., PENTA M.C. & CAVAGNA G.A. (1995) Energy-saving gait mechanics with head-supported loads. Nature, 375: 52-55
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