Animal locomotion results from interactions of rhythmic movements of the body with the environment. A biomechanical model for the movements may yield an unstable periodic orbit of a dynamical system. In these circumstances, the animal must exercise active control to maintain stability. This lecture will discuss the problem of estimating the Floquet multipliers which characterize local stability properties of a periodic orbit. The information needed depends upon perturbations from the periodic orbit. Animals can either rely upon fluctuations in the environment (''noise'') or generate excitations that move the organism off the orbit to obtain the required information. We discuss these alternatives in terms of the distribution of residuals between the animal's trajectory and the target periodic orbit for its motion.