Mapping the Pelvis - PSIS bones Download – This is an hour long mat class bringing observation to the PSIS bones. The PSIS bones are the back hip bones of the pelvis. This class emphasizes how the PSIS bones of the pelvis coordinate with the motions of the legs. You will also learn how the PSIS bones mobilize in relationship to the spinal motions such as arching, curling, rotating and side-bending.