Reaction to disturbances of a walking leg during stance

The ground reaction forces exerted by the legs of freely walking stick inse cts, Carausius morosus, were recorded during normal and perturbed locomotio n. The animals walked along a path into which a three-dimensional force tra nsducer was integrated. The transducer registered all three components of t he 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 v ariable affecting the forces and the trajectories of leg movements during u ndisturbed 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 pl atform in various directions within the horizontal plane and at two differe nt rates. Perturbations were applied either immediately after leg contact o r after a delay of 300 ms. The reactions to these disturbances were compati ble 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 t hat provides a combination of negative and positive feedback control mechan isms to resolve the apparent discrepancies between our results and those of previous studies.