263 35. STRUCTURAL IDENTIFICATION OF THE LUNG-THORAX SYSTEM Ch. Depeursinge Federal Institute of Technology, Lausanne, Switzerland 1. INTRODUCTION 1.1. Functional identification Functional identification of the lung-thorax system can be achieved by a stimulus-response experiment : in the so called "forced excitation" (1) or "forced oscillation technique" (2), a pressure signal is applied to the mouth and acts as a stimulus. The airflow induced by the pressure is the response of the lung- thorax system. If the system behaves approximately linearly, the functional relationship between the stimulus and the response can be described by a transfer function, which, in this particular case, is the acoustical impedance of the lung-thorax system. This transfer function is usually estimated in the frequency domain. Most of the early experiments have been performed with sinusoidal excitation at a single frequency or at a limited number of frequencies. Non-sinusoidal stimulations have been used in later experiments. Signals containing all the frequencies of interest allow a rapid determination of the impedance by spectral analysis of input and output signals. The use of random noise excitation presents further advantages : Gaussian white noise excitation yields the most comprehensive input signal to the test system. Following Wiener theory (3), the non-linear behavior of a physio- logical system can be described by its response to a Gaussian white noise stimulation. The set of Wiener kernels achieves a complete characterization of the dynamic mechanical behavior of the lung-thorax system (4). This "black box" approach ignores, however, the mechanisms underlying the dynamic response of the lung-thorax system. One S. M. Perren et al. (eds.), Biomechanics: Current Interdisciplinary Research © Martinus Nijhoff Publishers, Dordrecht 1985