Controlling a system with redundant degrees of freedom: II. Solution of the force distribution problem without a body model.

What strategies may insects use when controlling redundant degrees of freedom? We investigate this question in standing stick insects. Specifically, the question is addressed how the changes of the torques are coordinated that are produced by the 18 leg joints in a still standing animal. Using a generalization of the principal component analysis, three coordination rules have been identified. These rules are sufficient to describe more than half of the variation observed in the data. To move from a descriptive approach to hypotheses on how the neuronal system may be structured, two simulation approaches are proposed. In both cases, torques are decreased by randomly selected values. In the first simulation, the coordination rules derived from the principal components are used to produce changes in torques. In the second simulation, the individual joint torques are modified using a simple local approach. In both approaches, the resulting torques are re-adjusted by Integral controllers applied in each joint. The results show that the torque distribution problem can be solved by a local approach without requiring a body model.