This paper presents a three dimensional computational model to simulate the mechanical behavior of the muscle-tendon unit (MTU). The model has been formulated through a strain energy density function for hyperelastic materials incorporating both the passive and active behavior of the connective tissues, and taking into account preferential directions for both behaviours, not necessarily coincident. The initial stresses, usually present in soft tissues, have been considered when performing the MTU simulation. The geometry used corresponds to the rat tibialis anterior muscle reconstructed from «in-vivo » magnetic resonance images. This paper also studies the effect of the fascia on the MTU passive behaviour and the setting of a hyperelastic model for this tissue by means of experimental tests.