Fig. 1. A hypothetical diagram of quadrupedal walking. The fore quarters and hind quarters are represented as independent inverted pendulums. If the mass distribution is equal between the fore and hind quarters and the limbs cycle at evenly spaced time intervals, the pendular movements of the fore quarters and hind quarters offset each other. When the fore quarters are highest (i.e. gravitational potential energy is maximum), the hind quarters are lowest. Similarly, when the fore quarters are moving fastest (i.e. maximum kinetic energy), the hind quarters are moving slowest. As a result, the gravitational potential energy (E_p) and kinetic energy (E_k) are constant throughout the stride. Bars indicate foot–ground contact times, and the footfall order is left hind (LH), left fore (LF), right hind (RH) and right fore (RF) limb. COM, center of mass.