  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 $\alpha_s(\mu^2)$ 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 $\alpha_V(Q^2)$, the coupling in the heavy quark potential,
which can be compared with lattice gauge theory predictions. The corresponding
QED results for $\tau$ pair production allow for a measurement of the magnetic
moment of the $\tau$ and could be tested at a future $\tau$-charm factory.
