Long-distance behavior of the invariant charge in non-Abelian gauge theories

The long-distane behavior of the invariant charge is investigated. For massive quarks, the gluon--quark--antiquark charge is equal to the massless Yang--Mills charge. This result reflects the fact that the infrared limit is reached only as the gluon and quarks approach their mass shell at the same rate. A nonperturbative scheme in terms of the invariant charge, that could explicitly display the infrared structure of non-Abelian theories, is proposed