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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 (Mf).
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 Mf 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. 
zcom 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.
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