  Several locomotion strategies are based on the anisotropic nature of the
forces experienced by the moving body with its environment. We report
experiments on the anisotropy of the frictional force experienced by a cylinder
moving in a granular medium as a function of the orientation $\alpha$ between
the cylinder and its velocity. The component of the force in the direction
parallel to the velocity of the cylinder is always higher than the
perpendicular component and the force is therefore anisotropic. While the
parallel component increases continuously with the cylinder angle $\alpha$, we
observe that the perpendicular component reaches a maximum value for an
orientation of $\alpha_c \simeq 35^\circ$. In order to rationalise these
observations, we have developed a theoretical model which assumes that the
mechanical energy required to move the cylinder is dissipated by friction and
establishes a relationship between the parallel and perpendicular force
components that is consistent with experiments.
