Reaction to disturbances of a walking leg during stance.

The ground reaction forces exerted by the legs of freely walking stick insects, Carausius morosus, were recorded during normal and perturbed locomotion. The animals walked along a path into which a three-dimensional force transducer was integrated. The transducer registered all three components of the forces produced by a single leg when, by chance, it walked on the force platform. The stiffness of the walking surface was found to be a critical variable affecting the forces and the trajectories of leg movements during undisturbed walking. The forces produced by a leg were considerably smaller and the trajectories were closer to the body during walking on soft versus stiff surfaces. Perturbations during stance were generated by moving the platform in various directions within the horizontal plane and at two different rates. Perturbations were applied either immediately after leg contact or after a delay of 300 ms. The reactions to these disturbances were compatible with the hypothesis that the velocity of leg movement is under negative feedback control. This interpretation is also supported by comparison with simulations based upon other control schemes. We propose a model circuit that provides a combination of negative and positive feedback control mechanisms to resolve the apparent discrepancies between our results and those of previous studies.