FLEXORS: rectus abdominus, psoas(depending on position)
SIDE BENDERS (LATERAL FLEXION): quadratus lumborum
COMMENTS: When working on the back, the two most common muscles involved are the multifidus and the quadratus lumborum. The next most common muscle is the gluteus medius, which refers pain into the sacro-iliac joint. Working these muscles will usually help most back pain.