Avian Bipedal Locomotion
Kinematics & kinetics of long-axis rotation in avian bipedal locomotion
Published abstract (PDF,71KB).
Our current understanding of avian bipedalism is deeply rooted in a 2-D paradigm. Kinematic and kinetic analyses are typically limited to flexion/extension, but birds and other so-called 'erect' tetrapods do not really operate parasagittally. The goal of this project is to identify insufficiencies in a 2-D model by measuring 3-D movements and forces/moments in Helmeted Guineafowl (Numida meleagris) during steady and maneuvering locomotion using marker-based XROMM (Figure 1). We are focusing on long-axis rotation (LAR) because previously this degree of freedom has been impossible to measure accurately due to soft tissue artifacts.
Rather than using beads, we implant ~2.5-mm marker "cones" created from sharpening and scoring 0.8-mm diameter carbide steel rods (following Jenkins et al. (1988) Science. 241: 1495-1498). When mounted in a pin vise, the sharp tips can be inserted directly into the trabecular bone of the pelvis, femur, tibiotarsus, and tarsometatarsus without need for drilling or cement (see Figures 2-5 below). We are recording birds in the W.M. Keck XROMM Facility at 250 frames per second while they walk and turn within the confines of a small treadmill. Six degree of freedom joint kinematics are extracted from bone animations using an explicit Joint Coordinate System based on biomechanical standards for the human hip and knee.
Preliminary results reveal that both the femur and tibiotarsus undergo significant, repeatable cycles of LAR during steady walking (Movie 1). During turning, LAR at either the hip, knee, or both joints is used to reorient the foot with respect to the body. Tibiotarsal LAR can be dramatic, with rapid rotations of as much as 50° taking place at the knee.
To begin to decipher how the hind limb produces and accommodates non-planar loads, we are also measuring the passive range of motion at the hip, knee, and ankle with XROMM. A demuscled preparation of the marked limb is manipulated to map out the excursions permitted by the osseoligamentous tissues of each joint. Comparison of the actual poses used in vivo to the potential range of motion will help us discern when passive mechanisms are being used to control some degrees of freedom and when muscular input is required. Such insights will provide a new perspective on tetrapod joint design, intralimb coordination, and the evolution of locomotion in theropod dinosaurs.