Predictions for the angular distribution of massive quarks and
leptons are presented, including QCD and QED corrections. Recent
results for the fermionic part of the two-loop corrections to the
electromagnetic form factors are combined with the BLM scale fixing
prescription. Two distinctly different scales arise as arguments of
αs(μ2) near threshold: the relative momentum of the
quarks governing the soft gluon exchange responsible for the Coulomb
potential, and a large momentum scale approximately equal to twice
the quark mass for the corrections induced by transverse gluons.
Numerical predictions for charmed, bottom, and top quarks are given.
One obtains a direct determination of α\it\scriptsize
V(Q2), the coupling in the heavy quark potential, which can be
compared with lattice gauge theory predictions. The corresponding
QED results for τ pair production allow for a measurement of
the magnetic moment of the τ and could be tested at a future
τ-charm factory.