Perturbation measurements of papillary muscle elasticity

Small step-length perturbations (less than 0.5% Lmax) have been applied to rabbit papillary muscle for the purpose of measuring muscle stiffness at rest and during isometric twitch-force development. These stiffness-force measurements were fitted to four mechanical analogs to evaluate whether any of the models adequately predict the mechanical properties of papillary muscle. Stiffness-force relations have been measured at varied initial muscle lengths so that a broad range of applicability of an analog could be evaluated. This study shows that none of the four experimentally definable analogs predicts the stiffness-force responses of the muscle over a physiological range of initial muscle lengths. A two-segment nonhomogeneous analog has the same stiffness-force characteristics as a six-parameter lumped model but cannot be differentiated from the lumped model without a measurement of segmental variations. It is concluded that the passive elastic elements of papillary muscle cannot be controlled only by the manipulation of total muscle length so as to deduce cross-bridge mechanical properties from whole-muscle measurements.