  Different classical theories are commonly applied in various branches of
physics to describe the relativistic dynamics of electrons by coupled equations
for the orbital motion and spin precession. Exemplarily, we benchmark the
Frenkel model and the classical Foldy-Wouthuysen model with spin-dependent
forces (Stern-Gerlach forces) to the quantum dynamics as predicted by the Dirac
equation. Both classical theories can lead to different or even contradicting
predictions how the Stern-Gerlach forces modify the electron's orbital motion,
when the electron moves in strong electromagnetic field configurations of
emerging high-intensity laser facilities. In this way, one may evaluate the
validity and identify the limits of these classical theories via a comparison
with possible experiments to provide a proper description of spin-induced
dynamics. Our results indicate that the Foldy-Wouhuysen model is qualitatively
in better agreement with the Dirac theory than the widely used Frenkel model.
