Artifactual Motility of the Subcoxal Axis in a Model Insect Leg with Skew Joint Axes

Kinematic reconstructions of joint angles during walking in stick insects revealed that the subcoxal axis (SCA) was neither firm during the step (it was movable), nor was its position matched to the line between articulation condyles. It has been demonstrated on kinematic models of a leg that the application of a simplified leg model for reconstruction caused an artifactual result: apparent movement of SCA and its drift from the real firm position. Simplified models assume that the plane where leg segments lie rotates about the SCA or at least the axis of joint rotation is perpendicular to the leg segment. Real legs have joints with non-parallel or non-orthogonal axes. Models considered in this article had an oblique arrangement of joint axes. Real or artifactual motility of CAS is discussed.