  We compare and synthesize the results of two microrheological experiments on
the cytoskeleton of single cells. In the first one, the creep function J(t) of
a cell stretched between two glass plates is measured after applying a constant
force step. In the second one, a micrometric bead specifically bound to
transmembrane receptors is driven by an oscillating optical trap, and the
viscoelastic coefficient $G_e(\omega)$ is retrieved. Both $J(t)$ and
$G_e(\omega)$ exhibit power law behavior: $J(t)= A(t/t_0)^\alpha$ and $\bar
G_e(\omega)\bar = G_0 (\omega/\omega_0)^\alpha$, with the same exponent
$\alpha\approx 0.2$. This power law behavior is very robust ; $\alpha$ is
distributed over a narrow range, and shows almost no dependance on the cell
type, on the nature of the protein complex which transmits the mechanical
stress, nor on the typical length scale of the experiment. On the contrary, the
prefactors $A_0$ and $G_0$appear very sensitive to these parameters. Whereas
the exponents $\alpha$ are normally distributed over the cell population, the
prefactors $A_0$ and $G_0$ follow a log-normal repartition. These results are
compared with other data published in the litterature. We propose a global
interpretation, based on a semi-phenomenological model, which involves a broad
distribution of relaxation times in the system. The model predicts the power
law behavior and the statistical repartition of the mechanical parameters, as
experimentally observed for the cells. Moreover, it leads to an estimate of the
largest response time in the cytoskeletal network: $\tau_m \approx 1000$ s.
