Dynamic response of the isolated passive rat diaphragm strip

To further our understanding of the mechanisms underlying chest wall mechanics, we investigated the dynamic response of the isolated passive rat diaphragm strip. Stress adaptation of the tissue was measured from 0.05 to 60 s after subjecting the strips to strain steps of normalized strain amplitudes from 0.005 to 0.04. The tissue resistance (R), elastance (E), and hysteresivity (eta) were measured in the same range of amplitudes by sinusoidally straining the strip at frequencies from 0.03125 to 10 Hz. The stress (T) depended exponentially on the strain (epsilon) and relaxed and recovered linearly with the logarithm of time. E increased linearly with the logarithm of frequency and decreased with increasing amplitude. R fell hyperbolically with frequency and showed an amplitude dependence similar to that of E. To interpret the strong nonlinear behavior, we extended the viscoelastic model of Hildebrandt (J. Appl. Physiol. 28: 365-372, 1970) to include an exponential stress-strain relationship. Accordingly, the step response was described by T - Tr = Tr(e alpha delta epsilon - 1)(1 - gamma log t), where delta epsilon is the strain amplitude, Tr is the initial operating stress, alpha is a measure of the stress-strain nonlinearity, and gamma is the rate of stress adaptation. The oscillatory response of the model was computed by applying Fung's quasi-linear viscoelastic theory. This quasi-linear viscoelastic model fitted the step and oscillatory data fairly well but only if alpha depended negatively on delta epsilon, as might be expected in a plastic material.