Fig. 5. Vertical displacement of the fore pendulum, hind pendulum and system center of mass versus time for specific examples from the two-pendulum model of quadrupedal walking. The model consists of two identical independent pendulums representing a quadruped's fore and hind quarters with a variable phase relationship () and mass distribution (M_f). is the percentage of stride time that the fore pendulum displacement lags the hind pendulum displacement (analogous to the phase between a dog's ipsilateral fore and hind limbs), and M_f is the ratio of fore pendulum mass to the combined mass of both pendulums (equivalent to the fraction of body weight supported by the fore limbs). The time interval shown is equivalent to a complete dog stride. z_com is the magnitude of the center of mass vertical displacement relative to the pendulum displacement, and is the time interval between the peak center of mass vertical displacement and the peak hind pendulum vertical displacement, expressed as a percentage of time between peak hind and fore pendulum displacements. A represents the pendulum phase and mass distribution combination actually used by the dogs, and D represents the phase and mass distribution shown in the hypothetical diagram of Fig. 1. B and C represent intermediate patterns.