We understand little about how animals adjust locomotor behaviour to negotiate uneven terrain. The mechanical demands and constraints of such behaviours likely differ from uniform terrain locomotion. Here we investigated how common pheasants negotiate visible obstacles with heights from 10 to 50% of leg length. Our goal was to determine the neuro-mechanical strategies used to achieve robust stability, and address whether strategies vary with obstacle height. We found that control of landing conditions was crucial for minimising fluctuations in stance leg loading and work in uneven terrain. Variation in touchdown leg angle (θTD) was correlated with the orientation of ground force during stance, and the angle between the leg and body velocity vector at touchdown (βTD) was correlated with net limb work. Pheasants actively targeted obstacles to control body velocity and leg posture at touchdown to achieve nearly steady dynamics on the obstacle step. In the approach step to an obstacle, the birds produced net positive limb work to launch themselves upward. On the obstacle, body dynamics were similar to uniform terrain. Pheasants also increased swing leg retraction velocity during obstacle negotiation, which we suggest is an active strategy to minimise fluctuations in peak force and leg posture in uneven terrain. Thus, pheasants appear to achieve robustly stable locomotion through a combination of path planning using visual feedback and active adjustment of leg swing dynamics to control landing conditions. We suggest that strategies for robust stability are context specific, depending on the quality of sensory feedback available, especially visual input.
This work is supported by a grant from the Biotechnology and Biological Sciences Research Council (BB/H005838/1) to M.A.D.
LIST OF SYMBOLS AND ABBREVIATIONS
- centre of mass of the body
- total mechanical energy of the body CoM (sum of EP and EK)
- kinetic energy of the CoM
- gravitational potential energy of the CoM
- ground reaction force
- sagittal impulse
- leg stiffness
- effective virtual leg length, measured between the CoM and the toe
- resting leg length measured as hip height during quiet standing
- body mass
- moment about the body CoM
- power of the CoM
- spring-loaded inverted pendulum
- work done on the CoM
- change in total energy of the CoM over a step cycle (implies net work done)
- velocity angle
- angle between the leg and body velocity vector
- sagittal impulse angle
- leg angle
- leg retraction velocity
- © 2012.