Consider the propagation of a crack through a matrix containing short fibres of length lc such that the fibres cannot break. The fibres will bridge the crack and for the crack to extend it is necessary to pull the fibres out of the matrix. Thus the stored elastic strain energy must do work pulling out the fibres against friction or by shearing the matrix parallel to the fibres as well as driving the crack through the matrix. We can estimate the work done pulling out a single fibre by integrating the product F(x).x (force x distance) over the distance lc/2, where F(x) is the force - distance equation given by the shear lag model.