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

In two-dimensional (2D) kinematic stud- ies, limb positions in three-dimensional (3D) space ob- served in lateral view are projected onto a 2D film plane. Elbow and knee-joint angles that are less than 208 out- of-plane of lateral-view cameras generally exhibit very little measurable difference from their 3D counterparts (Plagenhoef (1979) Environment, Behavior, and Morphol- ogy; New York: Gustav Fisher, p. 95-118). However, when limb segment angles are more than 208 out-of- plane, as is often the case in locomotor studies of arbor- eal primates, elbow and knee angles can appear signifi- cantly 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 trigonomet- ric transformations. Am J Phys Anthropol 000:000-000, 2006. V C 2005 Wiley-Liss, Inc.