The mechanical properties of soft body tissues J.C. Barbenel Interest in the mechanical properties of the soft body tissues has been revived by the development of haptic teaching and training models and the development of Finite Element Models for the stress analysis. The properties of the soft tissues are complicated, but the characteristic non-linear stress/strain behaviour and significant anisotropy should be the starting point of soft tissue modelling. The primary method of modelling the mechanical properties is to assume that they are complicated examples of the general class of non-linear materials that are hyperelastic. This assumption is, in part, driven by the well-established theory of hyperelastic materials and the pre-existence of stress analysis methods for such materials, both analytical and numerical. Examples of formulations of hyperelastic constitutive relations for the soft tissues and their limitations will be discussed. There is, however, no evidence that the soft tissues are hyperelastic materials. A simple method of determining whether the results of biaxial extension tests on materials can be described as hyperelastic will be described. The method was applied to rubber sheet, which appeared to be hyperelastic, and to bovine pericardium, which was not hyperelastic.