Killer shoes

Last updated 14:17 23/11/2009

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The loads carried by high-heeled shoes present the same sort of issues as house piles, and the health of women's backs and ankles are at stake.

Killer heels are not only a bold fashion statement, but they also kill many women's backs.

Take a late-night walk through the city centre and you will see large numbers of young women coming out of their favourite haunts dressed smartly, but walking barefoot and carrying a pair of high-heeled shoes.

"Quite a fashion statement," a design student recently told me.

She was not talking about the barefoot look, but was referring to the exaggerated curvature of the feminine lower leg, which gives the impression that the women concerned are walking on their toes.

As a mere male, I have always wondered why anyone would go to such extreme lengths to wear what is clearly not only very uncomfortable, but also very unstable, and apparently somewhat injurious to health.

My wife reminds me that women, and even some men, will go to extraordinary lengths to look good.

High heels are one of the most common causes of ankle fractures and back pain.

Many modern high-heeled shoes now have to be made with steel reinforcing, to prevent the heel from snapping under the wearer's weight imposed on it – an indication of the structural challenges posed by this unique footwear.

Talking of structural challenges, high heels pose even more subtle challenges than those well known in the construction industry.

Architects use the term "point loads" when referring to the forces acting on the piles under a house. All loads must eventually be carried to the ground.

The entire weight and, therefore, load of a house or building can be carried through the few narrow piles buried in the ground. If the total area of all the piles is 10 per cent of the total floor area of the house, the effective pressure (force per unit area) from gravity forces acting on the house (its own weight) on the piles in the ground is 10 times greater than if the house had its load evenly distributed throughout the floor area.

If the piles were made thinner, say 1 per cent of the total floor area, the pressure would become 100 times greater.

Using the high-heel example, when striving for thinner heels, the pressure caused by the wearer's weight increases logarithmically as the dimensions of the width of the heel reduces. Even if only one millimetre is shaved off the width and the depth of a high heel, it can double the pressure exerted by the wearer.

When walking over a wooden floor in high heels, women may have noticed that they leave a neat row of little dents in their wake. This is certainly not the way to retain the friendship of the owner of the floor.

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Structural engineers and architects have managed to overcome these point-load problems on piles by encasing the wooden piles inside very large cubes of concrete, effectively increasing the area under the ground and decreasing the point loads.

If one looks closely, many women's high-heel shoes flare slightly outwards where the heel meets the ground. This imitates the concrete encasing solution and spreads the load, thus providing a more stable base and reducing teetering. However, the fascination with very high heels has caused several experienced models to take a tumble on the catwalk, a work hazard, if ever there was one.

Orthopaedic surgeons warn that high heels worn over time can cause serious back problems by raising and tilting the pelvis to cause curvature of the spine.

The vertebrae in the back are thus subjected to significant increased point loads, placing an imbalance throughout the body. It is no small wonder that they are called killer heels.

David Shillington is head of UCOL's School of Applied Health Sciences.

- © Fairfax NZ News

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