The influence of the adipose tissue thickness on near-infrared spectroscopy to monitor the haemodynamics in muscles is investigated. It is shown by calculation of the mean optical pathlength of two-layered turbid media that the validity of the modified Beer-Lambert law is reduced with increasing thickness of the first layer (fat), because the assumption, that the mean optical pathlength is constant, is not fulfilled. In order to avoid these problems we demonstrate that it is possible to derive the optical properties of both layers from time-resolved reflectance measurements using a solution of the two-layered diffusion equation if the thickness of the first layer is known.