ALEXANDER, R.McN.: Stability and Manoeuvrability of Terrestrial Vertebrates

For a standing animal to be statically stable, a vertical line through its centre of mass must pass through the polygon of support defined by its feet. Statically stable gaits are possible for quadrupeds but do not seem to be used; a discussion of turtles shows why they may be disadvantageous. Physical and mathematical models have shown that passive bipedal walking machines can be dynamically stable for motion in the sagittal plane, but not in roll. Accelerations and decelerations of animals may be limited by muscle strength, by the coefficient of friction with the ground, or by considerations of stability. Cornering ability similarly may be limited by strength or by the coefficient of friction. A simple mathematical model shows that it may be faster to use a longer route with corners of larger radius,than a shorter one with sharper corners. The concepts of oversteer and understeer,as applied to wheeled vehicles, may throw light on the stability of running.