QUAD

QUAD animates the six different tripod-aits ("walk" or "crawl") that a quadruped is capable of.

There are 4 distinct stepping patterns. They are RB-LB-LF-RF, RB-LB-RF-LF, RB-LF-LB-RF and RB-RF-LB-LF All others are rotations or reflections.

Some of the gaits are less suitable than others. Two of the gaits can be made to be completely stable but neither of those are what an animal uses. The other four gaits are only marginally stable at some phases in the cycle but all four can be made to be at least marginally stable during the whole cycle by shifting the body side to side and/or forward/backwards.

In all gaits, successivefoot-down triangles abut or overlap so it is so it is always possible to make the gait (at least marginally) stable by, say, shifting the body weight around. But in three of the gaits, the animal would sometimes have to shift its CoG backwards to keep stability at least marginal. In two other gaits, successive triangles always overlap so it is possible for the CoG never to approach the edge of a triangle (and the CoG always moves forward). However, those gaits are not left/right symmetrical.

In the remaining gait (the one used by animals) successive triangles do not overlap. That gait is only marginally stable at two phases in the cycle.

So you'd think that an animal would choose one of the two gaits where the triangles overlap and the CoG is always within a triangle. It doesn't; it chooses a marginally stable gait. Why? In three of the gaits, the CoG would have to move backwards so they're unsuitable. In the two always-stable gaits, the triangles are sometimes quite small. In the gait used by animals, the triangle is always large. That's why it is the best gait. It seems to be nothing to do with marginal stability.

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