  We study the statistical force of a nonequilibrium environment on a
quasi-static probe. In the linear regime the isothermal work on the probe
equals the excess work for the medium to relax to its new steady condition with
displaced probe. Also the relative importance of reaction paths can be measured
via statistical forces, and from second order onwards the force on the probe
reveals information about nonequilibrium changes in the reactivity of the
medium. We also show that statistical forces for nonequilibrium media are
generally nonadditive, in contrast with the equilibrium situation. Both the
presence of non-thermodynamic corrections to the forces and their nonadditivity
put serious constraints on any formulation of nonequilibrium steady state
thermodynamics.
