  We investigate the displacements of a probe particle inside a glass, when a
strong external force is applied to the probe (active nonlinear microrheology).
Calculations within mode coupling theory are presented for glasses of hard
spheres and compared to Langevin and Brownian dynamics simulations. Under not
too strong forces where the probe remains trapped, the probe density
distribution becomes anisotropic. It is shifted towards the direction of the
force, develops an enhanced tail in that direction (signalled by a positive
skewness), and exhibits different variances along and perpendicular to the
force direction. A simple model of an harmonically trapped probe rationalizes
the low force limit, with strong strain softening setting in at forces of the
order of a few thermal energies per particle radius.
