NARMAX representation and identification of ankle dynamics

Representation and identification of a parallel pathway description of ankle dynamics as a model of the nonlinear autoregressive, moving average exogenous (NARMAX) class is considered. A nonlinear difference equation describing this ankle model is derived theoretically and shown to be of the NARMAX form. Identification methods for NARMAX models are applied to ankle dynamics and its properties investigated via continuous-time simulations of experimental conditions. Simulation results show that 1) the outputs of the NARMAX model match closely those generated using continuous-time methods and 2) NARMAX identification methods applied to ankle dynamics provide accurate discrete-time parameter estimates. Application of NARMAX identification to experimental human ankle data models with high cross-validation variance accounted for.