The purpose of the research proposed here is to develop a reasonably comprehensive description (constitutive law) of the biomechanical behaviour of heart muscle. This is a necessary first step in the development of mechanical analysis of the heart. From a clinical standpoint there is need to perform mechanical analysis of the heart to predict its performance (e.g.: the pump function) in health and in disease. Since the heart is a muscle-organ, its performance depends very much on its muscle behaviour under varying demands of every-day life. Ideally, therefore, a three-dimensional muscle-law is required to facilitate the mechanical analysis. Deduction of muscle-law (constitutive law) based on intact-heart experiments is difficult however, because interpretation of such data is complicated by geometrical, structural and material complexities. Additional aspects such as non-uniform organ contraction, interaction between chambers, tethering influence of the aorta, etc., also make data interpretation difficult. A reasonable alternative to this complex problem is first, to characterize the uniaxial behaviour of a muscle bundle and then use it to make deductions on the intact ventricle/heart. With this objective in mind, we aim to complete a program of experiments on active pig papillary muscles to obtain information leading to mathematical description of the mechanical behaviour. The experiments will be performed following new formulation based on the phenomenological concept of linear fading memory of the viscoelasticity theory. This framework turns out to be a general description of the muscle in which some of the well known results, such as Hill's hyperbolic force-velocity relationship can be identified as special cases. Similar work was done by us in the past, on passive (unactivated) muscle. This work resulted in a unified law for describing the passive muscle. In the work proposed here we hope to use similar methodology on active cardiac muscle.