I just did 2000 reviews in one day and let me tell you my butt is killing me!
I ended up backtracking a couple levels.
My own stupidity 
2 Likes
Yeah, I’d really not advise doing them in one sitting, because they’re pretty much all gonna come back in one go when they’re up for review again. I suggest doing only a few dozen or even a hundred at a time.
(Also, your back is 背中 rather than 後ろ)
4 Likes
for back i hear koshi, senaka and ushiro. My family are farmers and I help in the summer.
2 Likes
腰【こし】is specifically lower back
2 Likes
And butt is お尻 (おしり) which is a completely different thing…