  We analyze the propagating degrees of freedom in gravity models where the
scalar curvature in the action is replaced by a generic function $f(R)$ of the
curvature. That these gravity models are equivalent to Einstein's gravity with
an extra scalar field had previously been shown by applying a conformal
transformation. We confirm this result by calculating the particle propagators.
This provides further evidence of the unability of these models to explain the
accelerated expansion of the Universe without contradicting solar system
experiments.
