Heart valve leaflets are considered to be viscoelastic, much like most biological materials. When these materials are tested both uniaxially and biaxially, a so-called “preconditioning” effect is observed. Upon repetitive loading to a constant load, sucessive load/elongation plots migrate in the positive strain direction. After about 10 cycles, successive load/elongation plots are repeatable, both in terms of the energy dissipated as well as the exact elongation achieved at a given load. This type of preconditioning needs to be done prior to any data acquisition or analysis, since the initial curves are not reproducible from test to test. In other words, unless the tissue is preconditioned first, it will not generate the same load at the same strain.