Page last updated at 14:10 GMT, Wednesday, 24 June 2009 15:10 UK

Avoiding 'wheelies' slows animals


A greyhound demonstrates the movement in this slow-motion video

The acceleration of four-legged animals appears to be limited by their need to avoid tipping upward, thus losing traction as their legs miss the ground.

Previously it had been assumed that the acceleration was limited by the animals' muscle power alone.

But new research, published in Biology Letters, suggests that this "wheelie" avoidance is the limit at low speeds.

At higher speed, researchers say, acceleration is capped by the amount of power in the muscles.

"If your front end comes up - if you 'wheelie' - you then lose purchase with your front limbs on the ground," explained Sarah Williams of the University of Liverpool.

"If you then continue to accelerate you've got nothing preventing you from flipping over backwards.

"Co-workers working in a similar area had seen that lizards, when they accelerate, actually run bipedally (on two legs) and that brought around the theory that perhaps this 'wheelie-ing' might be limiting whether they could accelerate any further," she told BBC News.

Pitch perfect

Greyhound racing (J Usherwood)
Greyhounds can be seen to lose purchase on the ground after a start

Dr Williams and her colleagues at the Royal Veterinary College in London wondered if avoidance of this wheelie-ing phenomenon limited the acceleration of animals such as polo ponies and greyhounds.

The team took a number of measurements from ponies and greyhounds: the lengths of their legs, the distance from hip or shoulder to their centre of mass, and so on.

They then developed a mathematical model to match the "pitch avoidance" that they would undertake to keep them from wheelie-ing.

The team used high-speed camera footage of the animals to determine their speeds and accelerations, finding that the animals did not reach the theoretically possible accelerations when running.

"At those early stages of the acceleration... this pitch limit or wheelie-ing is preventing them doing so," Dr Williams explained.

However, this constraint did not operate once the animals got up to a reasonable speed.

"At higher speeds, the pitch limit appears to be overridden by an additional limit, that is the amount of power an animal can produce with its muscles - the power its engine can produce, essentially," she said.

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