The stress-strain curves of the fabric flexible composites and the soft tissues under various biaxial loads appear to be similar. They all contain curved fibers in soft matrices. Because of the curved fibers, inhomogeneous deformation occurs within the material. Most researchers believe that the uniaxial data cannot be used to characterize multiaxial behavior of the soft tissues in general. The constitutive equation obtained using data regression method usually gives a good description of the stress-strain behavior for the homogeneous material. It gives no physical meaning of these input values obtained for the soft tissues. Thus, these constitutive relations are highly dependent on the experimental procedure and cannot be generalized.