I think erector spinae, the long muscles going vertically up the side of the spine- the right side was much more tense and hard and even I could feel it protruded more than the left side, so the PT dug into that muscle hard. He told me to mimic that with a tennis ball at home, which I did that evening.
I don't know if it was the massaging, but the day after (yesterday) my back was really sore again. Like, fuk me, three steps backwards. Even today, left side, above and under hip bone still really sore. It hurts when I bend over (like gbg self suck stretch, which I've also been doing). Glutes? Sacrioliac? I have been doing a lot of squats, unweighted and with 10lb dumbbells.
One thing the PT's (there were two, one was probably a supervisor/teacher) prescribed which I had not come across, either googling or in this thread, was the knee lift. They noted that the pop up involves pulling your legs up under your torso. So they recommended, first do burpees one leg at a time since I told them I'm not ready to do them both legs. And then a knee lift with a resistance band. All the squats and deadlifts (when I eventually get to them) involve pushing with the legs, not pulling them up to the chest.
The knee to chest stretch is also the one that feels the best right now...
Discuss.