When a car or motorcycle accelerates really fast, like in a drag race, its front wheels can come off the ground. Wheelies are pretty sweet, but flipping over backward isn't, so drag-racing vehicles are designed to avoid the problem. A new study finds that four-footed speedsters have to deal with the same forces when they take off from a dead stop.
Most research on how animals run has focused on steady motions--running on a treadmill, for example, or flying in a wind tunnel. As high-speed video and other technologies have made it possible to study acceleration, research has focused mainly on muscle power, which has been seen as the main limit on gaining speed.
But researchers at the Royal Veterinary College just outside London wondered if four-legged critters face the wheelie problem. They came up with a simple mathematical model, published today in Biology Letters, to see how fast a quadruped could accelerate without tipping over backward. The model predicts that the longer the back is in relation to the legs, the less likely a dog is to flip over and the faster it can accelerate. Then the researchers tested the model by going down to the local track, London's Walthamstow Stadium, and video-recording individual greyhounds as they burst out of the gate in time trials. The acceleration approached--but never exceeded--the limit predicted by the model.
Next, the researchers tried the model in the much more glamorous world of polo. The horses have to stop and start rapidly during a match. For this study, the horses made quick starts and stops while carrying a device that measures acceleration and location. The model predicted their deceleration as well as acceleration. "It doesn't take a huge understanding of muscle physiology and mathematical modeling" to figure out how fast animals can accelerate, says co-author and zoologist James Usherwood. "It takes two measurements and the assumption that they don't want to flip over backwards." The wheelie problem is only an issue at low speeds; once animals get going, muscle power limits acceleration.
The model is a useful way to think about acceleration, says Harvard University comparative biomechanist Andrew Biewener. And testing it with animals that are bred and trained for competition solves a problem for people who study movement: not knowing if study subjects are working their hardest, notes Thomas Roberts, a biomechanist at Brown University. "In this case, you can be pretty sure that they got close to the actual musculoskeletal limit rather than the attitude limit."