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. Physical and mathematical models have shown that bipedal gaits can be dynamically stable. 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. It may be faster to use a longer route involving corners of larger radius than a shorter one with sharper corners.