 The cost of transport is used to compare the energetics of different gaits, as well as the gaits of different animals.In spite of the differences in leg number shown in terrestrial vertebrates, according to the inverted pendulum model of walking and spring-mass model of running, "walks" and "runs" are seen in animals with 2, 4, 6, or more legs.The term "gait" has even been applied to flying and swimming organisms that produce distinct patterns of wake vortices.