  We study the distribution of threshold forces at the depinning transition for
an elastic system of finite size, driven by an external force in a disordered
medium at zero temperature. Using the functional renormalization group (FRG)
technique, we compute the distribution of pinning forces in the quasi-static
limit. This distribution is universal up to two parameters, the average
critical force, and its width. We discuss possible definitions for threshold
forces in finite-size samples. We show how our results compare to the
distribution of the latter computed recently within a numerical simulation of
the so-called critical configuration.
