The polymer nematic liquid crystals can exhibit very high elastic anisotropy, due to their molecular conformation. Usually the twist constant K-22 can be much smaller (from similar to 0.1 to similar to 0.01 times) than the splay rigidity K-11, but also the contrary can happen. In these two cases an external field (either magnetic or electric), properly applied to a well aligned sample, is able to induce the formation of static stripes, due to a Freedericksz mechanism. In fact, the energetic cost of a mixed deformation is more favorable than the cost of the pure distortion involving only the bigger rigidity, as requested by the usual aperiodic Freedericksz effect. In the present paper, the critical conditions for the appearance of such kind of static stripes in polymer nematics are presented and discussed.