  We present a comparative study on two theoretical descriptions of
microrheological experiments. Using a generalized Langevin equation (GLE), we
analyze the origin of the power-law behavior of the main properties of a
viscoelastic medium. Then, we discuss the equivalence of the GLE with a
generalized Fokker-Planck equation (GFPE), and how more general GFPE's can be
derived from a thermo-kinetic formalism. These complementary theories lead to a
justification for the physical nature of the Hurst exponent of fractional
kinetics. Theory is compared with experiments.
