A sliding filament model for the activation and contraction of cardiac muscle is postulated in which a model parameter—the variable rate of cross-bridge attachment— represents the instantaneous degree of activation. Equations are formulated which describe the responses of this model to isometric and isotonic loading conditions. These equations allow one to calculate the time variation of the activation parameter when given the contraction time history, i.e., the time course of length and tension changes. This is accomplished numerically by treating the cross-bridge distribution function n( x, t) as a discrete function of x and t. The filament sliding that occurs between two successive evaluation of n and the value of n at the next time step are found by solving, by Newton's method, the equations representing the mechanical constraints prescribed by the loading conditions. We apply this numerical scheme with the model responses set equal to experimentally measured contraction time histories of isolated cardiac muscle. It is found that in order for the model and muscle response to match, the degree of activation of the muscle model must both increase and decrease with shortening.