It is well known that the structure of biological tissue is closely related to tissue functions and defines its viscoelastic properties. It is necessary to create a model combining structural organization of myocardium and its viscoelastic properties to develop a model of cardiac wall of intact or deceased heart. This paper is devoted to experimental and theoretical study of viscoelastic behavior of isolated myocardial samples. A three-dimensional structural-functional model of papillary muscle is presented. The model adequately describes nonlinear viscoelastic behavior of isolated papillary muscles under uniaxial strain both in static condition and under dynamic loading.