  We present a systematic study of the relativistic axial-vector four-current
distributions inside a nucleon. We show in particular that the slope of the
axial form factor $G_A(Q^2)$ in the forward limit -- conventionally denoted as
$R^2_A$ in the literature -- does not represent the three-dimensional
mean-square axial radius in the Breit frame, but corresponds instead to a
contribution to the mean-square spin radius. We derive explicit expressions for
the latter in different frames and find in general additional contributions
that depend on both the nucleon mass and the forward values of the axial-vector
form factors $G_A(0)$ and $G_P(0)$.
