  We propose a method to expand correlation functions with respect to the
spatial components of external momenta. From the coefficients of the expansion
it is possible to extract Lorentz-invariant form factors at zero spatial
momentum transfer avoiding model dependent extrapolations. These objects can be
profitably calculated on the lattice. We have explicitly checked the validity
of the proposed procedure by considering two-point correlators with insertions
of the axial current, the form factors of the semileptonic decay of
pseudoscalar mesons, and the hadronic vacuum polarization tensor entering, for
example, the lattice calculation of the anomalous magnetic moment of the muon.
