Identification of nonlinearities in the neuromuscular system

Techniques have been developed recently which yield nonparametric estimates of the dynamic linear and static nonlinear components of two simple cascade configurations: the Wiener system, consisting of a dynamic linear element followed by a static nonlinearity, and the Hammerstein system, made up of a static nonlinearity followed by a dynamic linear element. A description is given of the results obtained when these methods were used to identify important dynamic relations in the neuromuscular system. For the intact human ankle, the Hammerstein structure provided a good description of stretch reflex dynamics (the relation between joint position and EMG activity). In contrast, the stiffness of isolated muscle fibers (e.g. the dynamic relation between length and force) was modeled best using the Wiener configuration.<>