Polycrystalline materials are modelled using plastic trusses. The rods of the truss simulate the slip systems of a single crystal while the geometry of the truss controls the interaction between the different slip systems. For calculations of the evolution of the yield locus in the space of the nodal forces, i.e. in the generalized stress space, due to the geometric variation of the truss, (kinematic hardening) a ''direct'' formulation is looked for, in which only quantities of the stress space are involved. It is found that for an adequate approach, besides the nodal forces and the nodal velocities also moments and rotations of the rods have to be introduced. If this result would be transferred to a polycrystal then it would mean that kinematic hardening could be described, in a sensible way by means of a Cosserat continuum only. Comparing for a truss the yield loci of the direct formulation with yield loci calculated by an indirect method a good agreement is achieved.