  We discuss the usage of continuous external momenta for computing
renormalization factors as needed to renormalize operator matrix elements.
These kind of external momenta are encoded in special boundary conditions for
the fermion fields. The method allows to compute certain renormalization
factors on the lattice that would have been very difficult, if not impossible,
to compute with standard methods. As a result we give the renormalization group
invariant step scaling function for a twist-2 operator corresponding to the
average momentum of non-singlet quark densities.
