Energetical profit of the third segment in parasagittal legs

Abstract

The parasagittal walking legs are considered characteristic of mammals, birds and bipedal dinosaurs. The hind legs in all these animals, as well as the fore ones in quadrupedal mammals, include three segments, movable relative to the ground in the contact phase, arranged in a zigzag manner. Either scapula or tarsometatarsus was added as a third segment to the ancestral two-segment leg structure. The third segment is argued to be an energy-saving adaptation. It allows the reduction of the mechanical work of leg muscles against each other in the contact phase, in which the struggle is shown to be inalienable in locomotion over level ground at constant speed by means of voluntary choice of optimum segments’ kinematics. The would-be optimum kinematical pattern is deduced and appears to be rather close to that observed in reality. The energy-saving role of muscles crossing more than one joint is discussed. The absence of the third segment in the walking legs of the lower tetrapods, such as urodelans, lizards, etc, is argued to be associated with purely symmetrical locomotion in those animals. In this case the mechanical work of muscles against each other is kept low, owing to the opposite direction of the elbow and knee bends.