Chronic sitting keeps you from using your glutes or firing your core muscles. It also means your hip flexors and hamstrings are getting tight. All of this contributes to strain on other areas of the body. One of the muscles getting overworked is the quadratus lumborum (QL), which originates at the hip bone and attaches to the spine. This can leave you chronically tight and chronically sore.
This exercise was originally designed to stretch those QL muscles, but it has the added benefit of some isolated glute work (specifically the gluteus medius) at the top.
Lawnmower Back Extension
Your goal should NOT be to go heavy with this, but rather stretching and reaching (across the body) with a very slight, pain-free twist. Give it a shot.