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Running with greyhounds, horses, cheetahs… and dinosaurs
Locomotion research reveals winning edges and safer races
3 July 2009
'Power is nothing without control.' So read a marketing slogan for a well-known brand of sports clothing. But there is more behind the line than selling shoes.
BBSRC-funded scientists investigating the mechanics of movement have found that power under acceleration is limited by an animals’ need to avoid pitching nose-up, much like a wheelie on an over-revving motorcycle. Similarly, deceleration is limited by the need to avoid going tail-up and losing that energy by hitting the deck (ref 1).
Based at the Royal Veterinary College, the Structure and Motion Laboratory (SML) seeks to explain the majestic moves of some of the fastest and most powerful creatures on Earth, such as cheetahs and elephants, but the research can also be used to design safer race tracks for horses and greyhounds. It’s work that could even help human athletes perform record-breaking feats at the London 2012 Olympic Games.
GPS and inertia sensors atop riders’ helmets measures motion. Image: SML
The SML’s approach is to look not at a single animal or limb. Instead, it considers the forces and biomechanics of motion from individual muscle cells right up to the functioning of the whole organism. Researchers use an evolutionary perspective where necessary to simplify complex systems so that predictive models can describe the actions – and limits – of animal locomotion.
Sarah Williams at the SML has taken a similar, step-by-step approach to model two racing animals – greyhounds and polo ponies – as they sprint and slow.
"At low speeds an acceleration of high magnitude will push the front of the body upwards and that’s not particularly useful," Williams explains. "To accelerate stably they need all four feet on the ground, so this upward wheeling is limiting how readily they can accelerate."
To understand how this upward movement destabilises the animal, Williams and colleagues thus simplified the quadruped form to just two variables: the length of the legs and the length of the back. A ratio of the two figures relative to the centre of gravity then predicts the speed the animal can achieve before encountering this ‘pitch limit’.
"In greyhounds, below five metres per second they appear to be limited by pitch, beyond that muscle appears important. In polo ponies, it’s slightly lower at up to four metres per second they are constrained by pitch," says Williams. She goes on to explain that the speed difference is because of the different proportions of the two animals: greyhounds have longer backs in relation to their legs; ponies have longer legs in relation to their back.
At higher speeds pitch is no longer limiting. Instead, it is brute muscle power that limits further acceleration. A simple finding but not an obvious one, Williams says, until you find it. "What we didn’t know before was at what speeds each limit was apparent, or whether indeed either of them were actually limiting. So we were quite pleased when we saw that the measures from animal fitted the pitch-limit model and model for muscle power."
Williams adds that she’s interested in what limits animals’ performance so that information can be used to understand how animals get injured, and how we can prevent injury and improve animal welfare.
Her colleague James Usherwood also likes the approach. "As a starter for ten this model is remarkably close." He says the research might yield information about the best surfaces to use to reduce injuries.
"There’s always a question on athletic surfaces – race track, dog track – quite where you have the compromise between grip and absorbing the shock energies of feet hitting the ground."
These dogs do not slow up for corners. Image: SML
If the surface is too soft, racing itself becomes difficult and competition devalued because too much energy is lost to the playing surface. On the other foot, a harder surface leads to heavier impacts through the bones, tendons and muscles, increasing the chance of injury.
Usherwood suggests the prevailing view that more grip is always better could be wrong. "There’s a tendency, which breaks humans as well as animals, to give infinite amounts of grip, because that’s something you can design materials for," he says. "Perhaps this tells us that you don’t need to give them too much grip if they are limited by something that isn’t grip."
Life in the slow lane
The laboratory’s research explores the limits of what is physically possible. And it has thrown up some strange answers. Take humans running around a track for example. For years it had been suspected that there was a bias against competitors drawn in the inside lanes for short sprint races. Some thought that this was theoretical; others psychological.
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© Structure and Motion Laboratory, The Royal Veterinary College
Usherwood’s research confirmed that in the indoor 200m sprint, where the bends are tighter, more foot-to-track contact time was needed to counter the increasing centripetal forces acting on limbs in the inside lanes. The only way athletes could do this was by reducing stride length which slowed them down (ref 2). The International Association of Athletics Federations dropped the event after the 2004 World Indoor Championships.
Bizarrely, the same does not hold for four-legged animals such as greyhounds that can run around bends without slowing down. Usherwood was relating the story of how inside sprinters had to slow down in a bar and was challenged by a local who said his greyhound didn’t slow up when cornering. "I didn’t believe him and so we took cameras to the local greyhound track and low-and-behold he was right," says Usherwood.
How do greyhounds manage such a feat? Unlike human sprinters, they power locomotion with a hindlimb torque centred on the hips - just like the power coming from the back wheel of a bike. So, just as with bikes going round a velodrome, greyhounds can withstand a considerable 65% increase in forces without slowing. Scientists think it’s analogous to cycling because the muscles that supply power are separate from the ones that support weight (ref 3).
Usherwood adds that it’s not clear if this applies to other dogs. "We don’t even know if it’s particularly true for horses, but seeing as their acceleration is limited by the same physics as greyhounds, we suspect they are."
A run for your money
Could cheetahs, who can run 50% faster than greyhounds and racing horses, be using a similar trick? "We don’t know why the cheetah is faster," says research group leader Professor Alan Wilson. "Is it just a little bit better in every way, or does it have fundamental differences in the way it runs that mean it is so much quicker?"
At Whipsnade Zoo, Wilson’s team are observing cheetahs chasing food and combining ultra-high-speed photography and computer analysis to find out how they achieve this feat.
Motion research can improve animal welfare. Image: SML
Wilson is keen to stress that although his group looks at animal locomotion in the broadest sense – even wondering if dinosaurs like Tyrannosaurus rex could run (ref 4) – there are real applications from the research. "We’re using it to look at dairy cattle to see which ones are lame because lameness is a major welfare problem with dairy cattle," he says. "We’re looking at people who have lost limbs and the problems they have and how prosthetic limbs can help."
Wilson’s colleague John Hutchinson has also used Hollywood-style motion capture cameras combined with MRI and CT scans to build 3D computer models of elephant locomotion to show the stresses at work on their muscles, tendons and bones.
A better understanding of elephant biomechanics offers the possibility to spot injuries early. For example, older individuals are prone to osteomyelitis and arthritis. If these conditions are not treated early they can result in an elephant being put down, a scenario well worth avoiding.
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- Williams, S. B., Tan, H., Usherwood, J.R. and Wilson, A.M. (2009). Pitch then power: limitations to acceleration in quadrupeds. Biology Letters http://dx.doi.org/10.1098/rsbl.2009.0360 (external link)
- Usherwood, J.R. and Wilson, A.M. (2005). Accounting for elite indoor 200 m sprint results. Biology Letters http://dx.doi.org/10.1098/rsbl.2005.0399 (external link)
- Usherwood, J.R. and Wilson, A.M. (2005). No force limit on greyhound sprint speed. Nature 438:753-754. doi: http://dx.doi.org/10.1038/438753a (external link)
- Hutchinson, J.R., V. Ng-Thow-Hing, F.C. Anderson. (2007). A 3D interactive method for estimating body segmental parameters in animals: application to the turning and running performance of Tyrannosaurus rex. Journal of Theoretical Biology 246:660-680
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