I Didn’t Know That . . .

Q: A dog has very short legs. How can it run faster than humans?

A: The answer is complicated, according to physiologist David Carrier of the University of Utah. Running speed is a product of both stride length and stride frequency. Humans have a long stride length, but their stride frequency is much lower than a dog’s. In part, that is because a dog’s muscles contract more swiftly. More important, however, the dog’s leg has evolved to minimize the energy needed to accelerate and decelerate while running. Proportionately, a dog’s leg is much thinner than a human’s and its foot is much smaller, so much less energy is required to accelerate. Muscles also are closer to the body, primarily in the thigh, increasing their effectiveness. Horses, gazelles and other animals evolved for running exhibit this specialized evolution to an even greater degree, with extremely thin legs and small hooves.

Finally, dogs get extra speed because they gallop. If you observe a racing greyhound, for example, you’ll notice that his back bends so that the rear legs are well underneath the body. Uncoiling the back then provides a tremendous boost to the rear legs, increasing both speed and efficiency.