  For soft matter systems strongly driven by stationary flow, we discuss an
extended fluctuation-dissipation theorem (FDT). Beyond the linear response
regime, the FDT for the stress acquires an additional contribution involving
the observable that is conjugate to the strain rate with respect to the
dissipation function. This extended FDT is evaluated both analytically for
Rouse polymers and in numerical simulations for colloidal suspensions. More
generally, our results suggest an extension of Onsager's regression principle
to nonequilibrium steady states.
