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Neuromechanical simulations in the study of Cat and Salamander locomotion
Nalin I. Harischandra
This talk will present you two simulation studies of neural control of locomotion: on Cat and Salamander. Neurophysiological experiments in walking cats have shown that a number of neural control mechanisms are involved in regulating the movements of the hind legs during locomotion. It is experimentally hard to isolate individual mechanisms without disrupting the natural walking pattern and we therefore introduce a different approach where we use a model to identify what control is necessary to maintain stability in the musculo-skeletal system. We developed a computer simulation model of the cat hind legs in which the movements of the each leg are controlled by eight limb muscles whose activations follow a centrally generated pattern with no proprioceptive feedback. All linear transfer functions, from each muscle activation to each joint angle, were identified using the responses of the joint angles to an impulse in the muscle activation at several postures of the leg covering the entire step cycle. We analyze the sensitivity and stability of the each muscle action over the joint angles by using identified system transfer functions and their gain and pole plots. We found that the actions of most of the hindlimb muscles over the joints display inherent stability during stepping, even without the involvement of any proprioceptive feedback mechanisms and those musculo-skeletal systems are acting in the critically damped condition which gives them the ability of quick actining, which may important in evolutionary point of view, without becoming unstable.
Moreover, computer simulations have been used to investigate several aspects of locomotion in salamanders as well. Here, we introduce a three-dimensional (3D) biophysically realistic, forward dynamics model of a salamander in a simulated environment. The simulator is programmed using Python scripts with OpenGL wrappers for 3D graphics and Open Dynamics Engine wrappers (pyODE) for rigid body dynamics and prescribed neural output patterns are used to drive the activation of a set of lineraly modeled skeletal muscles. The model was successfully used to simulate two different gaits: walking and trotting on level ground and swimming in water. In fact, this is the first study that simulates the "walking gait"
with a traveling wave of activity in the axial muscles between the girdles on a salamander like locomotor model. In a separate experiment, the model was used to compare its ability to turn while walking by bending the trunk or by using side-steps in front legs. This is the first study in silico that explores the use of side stepping for turning in a salamander like locomotor model. We found that for turning, use of side-stepping or combination is more effective than use of trunk bending alone. However, to make a conclusion on the usage of side-stepping in real animals we suggest more physiological and kinematic experiments on turning behavior.