Analysis of a Four-Legged Walk
A four-legged walk is very similar to the two-legged variety, but multiplied by two. The creature's legs still rock back and forth at the hips, but the upper-body motion happens parallel to the ground rather than perpendicular to it. Whereas human shoulders rock back and forth in the vertical axis, a dog's "shoulders" will rock back and forth horizontal to the ground as the front paws move back and forth.
A four-legged walk is very similar to a two-legged walk in that the hips and shoulders have rotations that mirror each other. When the right hip is forward, the left shoulder is back, and vice versa. This action usually varies a bit because the front and back legs might be offset by a few frames. Notice how the spine curves much like a human's and that the left shoulder and leg are back, mirroring the hip pose, as in Figure 6. This means that the left front leg, too, is about to plant.
Figure 6 This step has the right rear leg forward; the horse is about to plant the foot.
As the legs move forward through the step, the legs that are not currently planted on the ground (the free legs) move forward. The rear legs are fairly similar to a human's, bending at the knee in much the same fashion. The front legs, however, are actually jointed so that they bend forward, much like a bird's. This dictates a slightly different lift motion for the front legs (see Figure 7). At this point, the spine is straight when viewed from the top, but it might bow or arch a bit more when viewed from the side. This will be character-dependenta dilapidated horse's back could sag quite a bit.
Figure 7 Halfway through the step, the free legs are moving forward. Notice how the front leg's joint causes a different bend in the leg.
The legs then move through the step and plant the free feet, repeating the first step. In addition to this, a four-legged animal can have several different gaits: the walk, the trot, the canter, and the gallop. The animal will vary the timing and rhythm of its steps as it moves faster and faster. In the walk, the animal's legs behave very much like the arms and legs of a humanif the right rear leg is back, the right front leg is forward, with the opposite happening on the left. This changes as the strides change, however. By the time the creature has reached full gallop, the front legs are in syncgoing forward and back nearly in unison, with the back legs operating as a mirror to the front.