SE 14-15 and lower back tightness

When I do SE 14-15 (pancake shoulder) I am strongly limited by tightness in the lower back, slightly towards the sides. Which muscles are the culprits? They don't come into play into pike work in FS (which is vaguely decent), or "central" pancake (which sucks but not as much as SE 14-15), but mostly when there is some twisting, for example the "non-central" parts (that is, when I lean towards one of the legs) of the various pancake walks, bounces etc. They are also particularly stiff after a cycling  session. And sometimes they feel stiff even after a normal day, e.g. when I bend to take something from the ground.

I definitely need to lightly stretch them every day for a while, but I am not sure about which muscles and which stretches. "Lateral" pancake work is clearly a possibility, but is there anything else which could be useful?

Thanks!

Obliques and especially quadratus lomborum. you can evaluate to sit on a box or bench, open your legs and do it with bent knees. very useful.

You may also want to look up QL walks on the forum. Great for mobilising this area prior to stretching.

Am I doing the QL walks right? I should probably have a straighter back...

Also @Alessandro Mainente is this what you meant?

Thanks!

The stretch showed it is not correct. if you carefully the video of the course you can see how the body should the flexd to the side and the same shoulder of the stretched side must be inside of the knee, while the other arm reaches upside the body. actually you are not showing what I cited above. the bent leg version takes off some tension but the focus of the exercise should be the same.

12 hours ago, Alessandro Mainente said:

The stretch showed it is not correct. if you carefully the video of the course you can see how the body should the flexd to the side and the same shoulder of the stretched side must be inside of the knee, while the other arm reaches upside the body. actually you are not showing what I cited above. the bent leg version takes off some tension but the focus of the exercise should be the same.

Thanks Alessandro, at about second 38 I try doing the stretch as in the course, I should have cut the video, sorry.

Yes probably the video stopped before, well at 38 second it is acceptable but you should open definitely more the legs since your cannot move inside the leg your shoulders, upper arm should be straight.