Muscles of the Lower-back
The muscles of the lower back are often overlooked by those studying voice and/or speech for the performer. While some of these muscles, the iliacus and the psoas major & minor, are primarily flexors of the lower limb (i.e. thigh and pelvis), the quadratus lumborum serves as the equivalent muscle of exhalation to the abdominals found in front. However, the psoas does interdigitate with the muscles of the diaphragm which arise from the lumbar vertebra, and I believe that it is wise to work on the psoas and increase its flexibility so that the diaphragm is not affected by it.
Connected to the crest of the pelvis in back (just outside the sacrum), the muscles course upwards, attaching to the transverse processes of the lumbar vertebra and to the twelfth rib and are quadrilateral in shape. They act to flex the lumbar spine toward the side that is working. In terms of respiration, the quadratus lumborum can be felt to stretch most easily when the abdominals in front are contracted and one "breathes into the lower back", allowing the diaphragm to push the viscera of the abdomen against these muscles. On contraction, they serve as muscles of exhalation. They may also hold the lowest part of the ribs in place during inhalation, allowing the diaphragm to drop down more effectively. Back to Respiration More on Anatomy The Physics of Breathing Application to the Performer