  Biological activity gives rise to non-equilibrium fluctuations in the
cytoplasm of cells; however, there are few methods to directly measure these
fluctuations. Using a reconstituted actin cytoskeleton, we show that the
bending dynamics of embedded microtubules can be used to probe local stress
fluctuations. We add myosin motors that drive the network out of equilibrium,
resulting in an increased amplitude and modified time-dependence of microtubule
bending fluctuations. We show that this behavior results from step-like forces
on the order of 10 pN driven by collective motor dynamics.
