Massage and stretching of the Quadratus Lumborum (QL) muscle is beneficial for singers.
“The QL originates via aponeurotic fibers into the iliolumbar ligament and the internal lip of the iliac crest for about 5 cm. It inserts from the lower border of the last rib for about half its length, and by four small tendons from the apices of the transverse processes of the upper four lumbar vertebrae. Occasionally a second portion of this muscle is found in front of the preceding. It arises from the upper borders of the transverse processes of the lower three or four lumbar vertebrae, and is inserted into the lower margin of the last rib. Anterior to the quadratus lumborum are the colon, the kidney, the psoas major and minor and the diaphragm.” (wikipedia anatomy)
The base of the ribcage and the floating ribs move when we inspire for singing and tension in the QL can restrict their free movement. Look at the pictures of the QL above and notice how much of it inserts into the bottom 12th rib in the back. I recently was having tension in one side of my ribcage on inhalation and tightness in the left lower quadrant of my back. Upon further inspection by a physical therapist, the culprit was a tense QL muscle on my left side. After deep tissue massage of the QL my breathing was fully restored and back pain gone. Keeping the QL flexible is important for singers as we need flexibility of our whole ribcage. Many muscles of the torso are also active during the “support of sound” and keeping a loose, flexible torso can assist the singer in not collecting too much tension in the core of the body. The QL fixes the 12 rib for support  in powerful belting or sustained loud singing and forced expiration. Click on the arrow to the right of the picture above to see some good stretches for the Quadratus Lumborum. Next time you see a masseuse have them massage the QL, especially its insertion to the 12 rib in the back. If you don’t have the luxury of getting a massage, rolling your waist on a plastic tube or using a tennis ball also works and feels fantastic.