The aim of this study was to calculate the theoretical variation of the non linear damping factor (B) as a function of the muscle shortening velocity. The theoretical variation of the B factor was determined from a muscle model which consists of a contractile component in parallel with a viscous damper both in series with an elastic component, and by using the characteristic equation of the force-velocity curve. In this muscle model, the viscous element modeled the inability of the muscle to generate as a big contracting force (while shortening) as possible under isometric conditions. The results show that the theoretical behaviour of the B factor was dependent on the shortening velocity and on the af parameter which varies according to the muscle fibre type composition and affects the curvature of the force-velocity curve.