The musculus erector spinae (back extensor) belongs to the back muscles and represents the musculature of the vertebral column. Many people suffer from complaints in that area so high importance can be attached to the musculus erector spinae.
The musculus quadratus lumborum is the deepest of the abdominal muscles and located at the posterior abdominal wall. It is also commonly referred to as a back muscle. The quadratus lumborum muscle is involved in the breathing as well as motions of the trunk.