The muscles to the sides of the lower back are the QL muscles. If this muscle goes into spasm, you can barely walk; get in or out of a chair or turn over in bed. You can also be pulled into all sorts of kinky side bending shapes.
The pain pattern when the QL goes awry is in the lower back and down into the hip region, which is quite similar to other muscle strain patterns. When you have a problem in this area, go through the lower back and hip stretches
The last group are on either side of the spine at the back. There are a whole bunch of them and collectively we call them the Erector Spinae Group or Para spinals. Like the others they are very important in maintaining the integrity of the curve in the lower back and of the posture of the body as a whole. When they are tight they tilt the pelvis forward which tends to sway the back. If they are slack you tend to have a rounded lower back and slouch. The Para spinals work closely with the Abbs. Tone in the Abbs keep the pelvis tilted back and the Para spinals tilt the pelvis forward. So you can see that getting optimal balance between these two groups is vital for individually optimised lower back function and general postural hygiene. So toning those up is one of the key components to developing good postural hygiene.