Dog Feet

In the real world, your feet work without you even thinking about them. You climb stairs, you turn in place, you ascend an inclined ramp, whatever. You don't think about where on your foot you impact the ground, how you roll to the ball, and then launch off the toe. But, if in an animation it's not done in the proper fashion, it looks like heck. I was told once by an instructor that a walk cycle is the hardest task an animator will have to deal with. Well, if you don't count object constraint animation, then yeah, I'd agree with that. It's a bite. But a good rig can take some of that bite out. I've done a few quadruped rigs before, but they've all be quite simple, and each time I pretty much got away with out having to make a working heel-ball-toe foot rig. This time, though...unh-huh. Puppy's gotta giddy-up, and trot, and roll over, and all the other assorted stuff that cute-wittle-puppies do.
Quads and Bis have pretty much the same joint hierarchy. In the images below you can see how the limb extrudes from the central spine by way of the scapula, humerus, ulna/radius, carpus, metacarpus, and then finally the phalanges. In the rear, well, you can follow the chain from the pelvis to the phalanges easily enough.

In you and me--I'm assuming you're a human--the order of the bones is the same. The scaling is different, and the necessity for a dog to put even weight on it's "arms" makes they dynamic of the "shoulders" work differently.

When you land on your step, the first pivot point is the heel--you've evolved differently than the dog has so there's more bone back there to handle your larger foot, seeings how you need to balance all-of-you on only two limbs. For sake of easier explanation, we'll consider this part of the Tarsals. From there, the next pivot point is between the metatarsals and the phalanges. When the dog's foot plants, though, it's more of a flat-plant, as there's no heel bone. There's a fleshy pad that a dog lands on--it's the human-equivalent to a tiptoe walk.
So, here I am, rigging this dog, looking at how the dog's bone structure is lined up, and how the dog is walking. Heck, why not used a modified human leg IK structure? Like this:
1. The toe pivot. Used for launching and for yaw pivots on weight shift.
2. Metatarsal and phalanges hinge. Used for tip-toe pivot, heel lift, and toe wiggles.
3. The imaginary heel. May not be on in there for real, but this allows me to land the heel down with the foot up.
4. The Shoulder, or in the rear it's the hip.

This method worked okay in this situation. I think, if I were to do it on a larger dog, I may go the route of the old-fashioned reverse foot setup.
I can't show the final product, obviously, because it's a work in progress, but I can show a test of the rig. Came out alright looking. Whatcha think?