2548 Introduction The vast majority of bipedal species are birds, which are thus valuable for understanding the biology of bipedal locomotion. Hind limb kinematics of terrestrial locomotion in birds has been used to study locomotor behavior (Reilly, 2000; Verstappen et al., 2000), muscle function (Clark and Alexander, 1975; Roberts and Scales, 2004), neural and motor control (Johnston and Bekoff, 1992; Johnston and Bekoff, 1996; Verstappen and Aerts, 2000), ontogeny of locomotion (Muir et al., 1996) and locomotor energetics (Fedak et al., 1982; Roberts et al., 1998; Roberts and Scales, 2004; Rubenson et al., 2004). Hind-limb kinematics of birds has also been used to explore the evolution of avian bipedalism and archosaur locomotion in general (Gatesy and Biewener, 1991; Gatesy, 1999; Hutchinson and Gatesy, 2000), and has been used to aid predictions of the locomotor abilities of extinct bipedal taxa (Hutchinson, 2004). A significant limitation of past kinematic studies, however, is the general reliance on two-dimensional (2-D) planar analyses. Analyses in 2-D have been found to be insufficient for understanding the movement of many terrestrial species. This is particularly apparent in those animals with sprawled postures, whose limb segments are not aligned in the sagittal plane [e.g. lizards and crocodylians (Gatesy, 1991; Reilly and Elias, 1998; Irschick and Jayne, 1999; Jayne and Irschick, 1999]. The importance of incorporating three-dimensional (3-D) kinematics is not restricted to sprawling gaits. Indeed, 3-D kinematics are necessary for revealing fundamental aspects of human locomotion, many of which have direct clinical application (Whittle, 1995), and some studies indicate that 3-D limb motions also occur in running birds (Cracraft, 1971; Gatesy, 1999), although no comprehensive joint analyses during locomotion have been undertaken. Examination of human kinematics has revealed that not only limb posture, but also the alignment of the joint axes, play an important role in determining the 3-D nature of limb movements (Piazza and Cavanagh, 2000; Besier et al., 2003; Most et al., 2004). Indeed, Piazza and Cavanagh (Piazza and Cavanagh, 2000) illustrated that a small misalignment between the assumed Although locomotor kinematics in walking and running birds have been examined in studies exploring many biological aspects of bipedalism, these studies have been largely limited to two-dimensional analyses. Incorporating a five-segment, 17 degree-of-freedom (d.f.) kinematic model of the ostrich hind limb developed from anatomical specimens, we quantified the three-dimensional (3-D) joint axis alignment and joint kinematics during running (at ~3.3·m·s –1 ) in the largest avian biped, the ostrich. Our analysis revealed that the majority of the segment motion during running in the ostrich occurs in flexion/extension. Importantly, however, the alignment of the average flexion/extension helical axes of the knee and ankle are rotated externally to the direction of travel (37° and 21°, respectively) so that pure flexion and extension at the knee will act to adduct and adbuct the tibiotarsus relative to the plane of movement, and pure flexion and extension at the ankle will act to abduct and adduct the tarsometatarsus relative to the plane of movement. This feature of the limb anatomy appears to provide the major lateral (non-sagittal) displacement of the lower limb necessary for steering the swinging limb clear of the stance limb and replaces what would otherwise require greater adduction/abduction and/or internal/external rotation, allowing for less complex joints, musculoskeletal geometry and neuromuscular control. Significant rotation about the joints’ non- flexion/extension axes nevertheless occurs over the running stride. In particular, hip abduction and knee internal/external and varus/valgus motion may further facilitate limb clearance during the swing phase, and substantial non-flexion/extension movement at the knee is also observed during stance. Measurement of 3-D segment and joint motion in birds will be aided by the use of functionally determined axes of rotation rather than assumed axes, proving important when interpreting the biomechanics and motor control of avian bipedalism. Key words: kinematics, ostrich, bird, three-dimensional, locomotion, joint. Summary The Journal of Experimental Biology 210, 2548-2562 Published by The Company of Biologists 2007 doi:10.1242/jeb.02792 Running in ostriches (Struthio camelus): three-dimensional joint axes alignment and joint kinematics Jonas Rubenson 1, *, David G. Lloyd 1 , Thor F. Besier 2 , Denham B. Heliams 3 and Paul A. Fournier 1 1 School of Human Movement and Exercise Science, The University of Western Australia, Crawley, WA, 6009, Australia, 2 Department of Orthopaedics, Stanford University, 341 Galvez St, Stanford, CA 94305, USA and 3 Fauna Technology, PO Box 558, Gosnells, WA, 6990, Australia *Author for correspondence (e-mail: jrubenson@csupomona.edu) Accepted 16 May 2007 THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY