Joint mechanics during movement : experimental and theoretical studies

In the first of three manuscripts, we describe a study in which we measured the electromyographic (EMG) and mechanical responses to brief stretches of the ankle plantarflexors applied at various instants during an imposed movement of the ankle. The results demonstrate that afferent input resulting from movement of the ankle can both modulate and inhibit the reflex EMG activity. They also reveal a dissociation of reflex EMG activity and reflex torque during the movement that cannot be explained on the basis of results obtained under stationary conditions.In the second and third manuscripts, we describe two new techniques to identify time-varying systems from ensemble data. These are meant to serve as building blocks in the development of an algorithm to identify joint mechanics during movement. The first technique applies to linear time-varying systems while the second serves to identify time-varying Hammerstein systems. The techniques can be used when the system varies quickly and require no a priori knowledge of the structure of the linear element and of the form of the time variation. Furthermore, the inputs do not need to be white. Simulation results demonstrate that the new techniques perform well in the presence of significant output noise with a reasonable number of realizations. These techniques are thus capable of yielding good system models under realistic conditions