Technical note: Out-of-plane angular correction based on a trigonometric function for use in two-dimensional kinematic studies

Abstract

In two-dimensional (2D) kinematic studies, limb positions in three-dimensional (3D) space observed in lateral view are projected onto a 2D film plane. Elbow and knee-joint angles that are less than 20° out-of-plane of lateral-view cameras generally exhibit very little measurable difference from their 3D counterparts (Plagenhoef [1979] Environment, Behavior, and Morphology; New York: Gustav Fisher, p. 95-118). However, when limb segment angles are more than 20° out-of-plane, as is often the case in locomotor studies of arboreal primates, elbow and knee angles can appear significantly more extended than they actually are. For this reason, a methodology is described that corrects 2D out-of-plane angular estimates using a series of trigonometric transformations. © 2005 Wiley-Liss, Inc.