> There may be other muscles involved

... indeed. The iliacus (sp?) and psoas are involved in a whole lot of complicated & important stuff in the hip region. Both run from the upper femur through the pelvic bowl; the iliacus anchors on the rear upper rim of the pevlic bowl and the psoas on the sides of the lower spine. The geometry is such that, if they're tight, these muscles can be stretched in both forward and backward bends and, if they're not flexible, cause strain & pain in the lower lumbar as a result.

I have this to some degree on my right side because of an old knee injury that caused a lot of tightness in my right thigh.

Stretches to directly address these: virabhadrasana (warrior) II, and variations thereof - focus on the upper front thigh of the back leg. Supta virasana (if your knees & ankles are up to it) or, gentler on knees & ankles but a more advanced backbend & psoas stretch, laghu vajrasana

Alan Little
 

Alan, I feel as if I had a yoga consultant always ready to answer my concerns whenever I see your comments here. I will start incorporating your suggestions into my daily practice. However, the supta virasana pose is well beyond my current flexibility. Mike