If you're feeling soreness in your lower back, in the area that feels like your kidneys, chances are that it's because you were training your lat muscles.
This puzzled me for years, but lately I've been trying to figure out the complex muscles of the back, and it has to do with the insertion points of your lats.
Your muscles have to be connected to your skeleton in order to work. Your lats insert into your skeleton at your armpits, your upper spine, and down by the top of your iliac crest (I had to look that up, it's the bone that kinda sticks up on the back top of your pelvis).
Exercises like lat pulldowns, or pull-ups on a bar, or any pulling movement above your head will train your lats. And in order for your lats to work, they have to be connected. And if you're like me, you mostly think of them as being connected by your armpits, and along your upper back. But of course they need to be connected down at the bottom, and that's the area that I always felt was my kidneys, and in reality it was just the bottom insertion point of my lats.
So no, you haven't done anything wrong. You are just feeling the effects of the weakest point of how that muscle works, where it tapers down to a small area, and connects with the bone. And those insertion points will always feel the strain more than the large area of the muscle belly, because, well, they're smaller.
Like a lot of guys, I've been training mostly my "mirror muscles" - that is, chest, arms, you know, stuff you can see in the mirror. But now I'm getting with the program and training my back muscles, and I am learning that you gotta know how they work, and what they do.
I hope this helps. By the way, like all muscle soreness, what it needs most is time. I won't be training this area directly until next Thursday, which is five days away, so in the meantime I'll baby the area, keep it warm, apply heat (both with my microwave heating pad and soaking in a hot bath), take my protein pills, eat healthy, drink plenty of water, and not worry so much.
|Back muscles. Note how the lats insert down by where you think it's your kidneys.|