  We present a strong coupling expansion that permits to develop analysis of
quantum field theory in the infrared limit. Application to a quartic massless
scalar field gives a massive spectrum and the propagator in this regime. We
extend the approach to a pure Yang-Mills theory obtaining analogous results.
The gluon propagator is compared satisfactorily with lattice results and
similarly for the spectrum. Comparison with experimental low energy spectrum of
QCD supports the view that $\sigma$ resonance is indeed a glueball. The gluon
propagator we obtained is finally used to formulate a low energy Lagrangian for
QCD that reduces to a Nambu-Jona-Lasinio model with all the parameters fixed by
those of the full theory.
