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Who's got more efficient legs, arthropods or humans?

Two Irish researchers have revealed that the legs of grasshoppers and crabs are ideally shaped to resist bending and compression, making the quality of their 'design' significantly superior to the blueprint for human legs. Writing in the Royal Society journal, the pair, fro...

Two Irish researchers have revealed that the legs of grasshoppers and crabs are ideally shaped to resist bending and compression, making the quality of their 'design' significantly superior to the blueprint for human legs. Writing in the Royal Society journal, the pair, from Trinity College Dublin, states that if human leg bones were built the same way, they could be twice as strong. 'Like all Arthropods, grasshoppers and crabs have so-called exoskeletons made from a very special material called cuticle,' comments one of the study authors, Professor David Taylor from the Trinity Centre for Bioengineering. 'This exoskeleton protects the animal like a knight's suit of armour. Recently we have shown that this cuticle is in fact one of the toughest natural materials.' 'In terms of evolution, having your bones on the outside has been a pretty good concept,' adds his co-author and partner, Dr Jan-Henning Dirks. '[For] millions of years, animals with exoskeletons such as insects, spiders and crustaceans [have been] found basically in every ecosystem in the world.' The researchers used the latest techniques from the fields of engineering mechanics, materials science and biomechanics to work out exactly why this exoskeleton is so successful. Their study put particular emphasis on the diameter and thickness of the bones. They used a special computed tomography machine to generate X-ray images of insect legs with a resolution of only a few thousands of a millimetre, and they collected and compared data from crabs and human bones. Their results show that while human leg bones have relatively thick-walled tubes, the legs of insects and crabs have a much thinner wall in relation to their radius. Professor Taylor comments: 'This relation of wall thickness to radius can tell us a lot about the mechanical stability of the structure. Imagine the bones as simple tubes. Now, if you had a limited amount of material, what would you do? Would you make a thin solid rod or a hollow, thin walled tube? When compressed, the rod might easily bend like a straw; the hollow tube, however, might buckle like a beer can.' The study shows that for a given weight, there is a mechanical optimal wall-thickness. For example, the leg shape of crabs represents an ideal compromise to resist both the bending and compression forces the crab experiences when walking under water. But the grasshopper leg is optimised to withstand the huge bending forces that come into play when it jumps. The study concludes that if somehow the human thigh-bone could be 'redesigned' as an exoskeleton that uses the same amount of bone material, it could be twice as strong as it is now. Professor Taylor says: 'Of course, there are numerous other factors determining the evolutionary advantages of endoskeletons and exoskeletons. However, we think that by taking a design engineer's view on the problem, we've been able to shed some light on the evolutionary development of skeletal forms.'For more information, please visit: Trinity College Dublin: http://www.tcd.ie/

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