Figure 1.

Human and ostrich hind-limb postures during mid-stance of running: (a,b) sagittal plane; (c,d) frontal plane. Note: only the right limb is displayed for the ostrich. The vertical line represents the orientation of the ground reaction force vector. The circular mark represents the position of the combined centre of mass for all lower limb segments of the right limb (excluding pelvis). Differences between limb structures include: (i) a more distal limb-mass distribution in humans compared with ostriches, (ii) a plantigrade posture where the metatarsal bones of the foot are kept in contact with the ground in humans compared with a digitigrade posture in ostriches (walking/running on toes), resulting in a longer effective limb length and an additional joint for storing and releasing elastic energy (tarsometatarso-phalangeal (TMP) joint), and (iii) shorter tendons crossing the distal joints (ankle and TMP) in humans compared with ostriches; tendons crossing these joints originate from muscle–tendon junctions close to the knee joint (and mid-shank for the human soleus). Images developed from Vicon BodyBuilder software (Oxford Metrics, Oxford, UK) and bone images from OpenSim software at corresponding limb postures (SimTK; www.simtk.org). For motion files of ostrich and human running, see electronic supplementary material.