If specifier_x is set to -fixed_in_space, the nodal points of the mesh remain fixed in space in x-direction. If a specifier_x is set to -follow_material, the nodal points of the mesh will follow material displacements in x-direction. The same holds for the other directions. In 1D, you only need to give specifier_x, etc.
Default each specifier is set to -follow_material.
This record options_mesh only is used if materi_velocity is initialised. If materi_displacement is initialized however, each specifier is automatically set to -fixed_in_space.