It's not just horses that gallop - if a dog is chasing after its favourite ball at full speed, it will be galloping too. When dogs walk or trot, the right and left legs do the same thing, using the same muscles to apply the same forces to the ground, at the same rate. However a gallop is different because all four legs have an individual role. To find out the different roles of the individual limbs during galloping, David Carrier and Rebecca Walter at the University of Utah enlisted a team of cooperative canines to sprint down a track, measuring their speed and the forces produced by the different limbs(p. 208).

When they are running flat out, dogs use a style of galloping called the rotary gallop. Just like walking, the limbs are placed on the ground in a certain order. Starting with the front legs, the so-called `trailing' leg lands first, followed by the `leading' leg, which lands further in front. The trailing and then the leading front legs are lifted off the ground so that the animal is airborne. Then, the trailing hind leg - which is on the opposite side to the trailing front leg - is placed on the ground followed by the leading hind leg. Once the trailing and then the leading hind legs have been lifted off the ground, the dog is airborne again, ready to start the cycle over.

They filmed the dogs at 250 frames s-1 sprinting down a 40 m runway, chasing a tennis ball, or encouraged by an experimenter holding a hot dog. `We wanted to see how they ran when they really went for it' says Walter;most of the dogs managed a top speed of around 10 m s-1, which gives Olympic sprinters a run for their money. A force plate embedded in the floor measured the vertical, braking and accelerating forces produced by each leg when it landed, and while it was on the ground.

Comparing the forces produced by the forelimbs, they found that the lead forelimb was on the ground for longer than the trailing forelimb, but that the average force produced by the lead forelimb was lower. This meant that the force produced over time - known as the force impulse - was the same for both legs. Examining the accelerating and decelerating forces showed that the trailing forelimb accelerated the body more while the lead forelimb decelerated it more. Turning their attention to the hindlimbs' forces, they found that the roles were reversed: the trailing hindlimb applied lower vertical force for a longer period of time, while the lead hindlimb produced greater accelerating forces. The results suggest that the trailing hindlimb and lead forelimb are not producing as much force as they should. `They could apply greater power, but it would probably be inefficient', Walter says. She suspects that the balance of forces between the legs needs to be just right so no energy is wasted during galloping. However, the differences in forces between the limbs could help dogs to turn when galloping.

The team also upturned one prediction about galloping. `People had predicted that the forelimbs would do more braking and the hindlimbs would do more acceleration' explains Walter, but the team found that both sets of limbs contributed equally to acceleration, while the forelimbs applied more deceleration that the hind limbs. Given that there were no major differences in the galloping styles of the different breeds, Walter thinks that her and Carrier's results will help researchers trying to learn more about galloping in four-legged creatures of all shapes and sizes.

References

Walter, R. M. and Carrier, D. R. (
2007
). Ground forces applied by galloping dogs.
J. Exp. Biol.
210
,
208
-216.