The hip flexors are attached to the front of the lumbar vertebrae and come down through the groin to attach to the thigh. They traverse the sacroiliac joints, where the sacrum joins to the pelvis, they also traverse the hip joints. These muscles have a massive influence area and when tight they compress the entire lower back, hip joints and pelvis together.
This muscle group gets noticeably tight when you sit for a time and then when you stand it is really stiff and sore in the lower back. You can’t stand up straight until you walk around for a while and the tight muscle eases off. Or if you are lying out straight and your lower back is sore but by bending your knees, the pain goes away. This muscle group is what pulls us into a stoop when we are older.
The hip flexors are the deep anterior muscles of the lower back and the other anterior muscles are the Abbs or abdominal group. We tend to lose Abb strength as time goes by especially after carrying and delivering babies or eating and drinking too much. When these muscles go slack your pelvis tips forward, your belly hangs out and you get more of a sway in our back.
This tends to compress the rear of the lumbar vertebrae together, disturbing normal mechanics and creating pain and inflammation. The Abbs are important in maintaining an integral relationship between the lower back and the pelvis. So generally speaking the Abbs need to be toned up to improve regional mechanics and help dissipate pain and ongoing problems.