Hey Anna 
Yeah, they are a pretty tough exercise if done correctly. I think one of the most common mistakes made in this exercise is not keeping the core engaged throughout the whole movement, as an example, treating the lowering of the legs as a kind of rest-the core does need to be engaged throughout.
I actually experience this myself at this time of year. If I don’t really engage my core my lower back often will rise off the ground, and this can definitely lead to aches and pains.
I took a quick video to demonstrate-first few reps I don’t keep my core engaged and relax during the lowering phase, the last few I engage my core throughout all the reps.
