Apples, oranges, and angles: Comparative kinematic analysis of disparate limbs

Tetrapod limbs exhibit diverse postures and movements during terrestrial locomotion. As with morphological traits, the history of kinematic evolution should be accessible to reconstruction through analysis of limb motion patterns in a phylogenetic framework. However, the angular data comprising most kinematic descriptions appear to suffer from limitations that preclude meaningful comparison among disparate species. Using simple planar models, we discuss how geometric constraints render joint and elevation angles independent of neither morphology, degree of crouch, nor one another during the stance phase of locomotion. The implicit null hypothesis of potential similarity is invalidated because angular data are not viably transferable among limbs of dissimilar proportion and/or degree of crouch. Overlooking or dismissing the effect of constraints on angular parameterization hampers efforts to quantitatively elucidate the evolution of locomotion. We advocate a search for alternative methods of measuring limb movement that can decouple intersegmental coordination from morphology and posture.