I think it might be useful to ask the question once more, and this time in English - and maybe try and put it more correctly.
I think the actual question is:
LEO translates "to arch one's back" in the sense of a cat arching its back - i.e. the arch is pointing upwards, and the result is basically the same as with "humping one's back".
OTOH, there are numerous sites on the web (like the examples I gave in my entry post), especially from the realm ot fitness, using the term, if I get things correctly, exactly in the opposite way, more or less advising the readers to do a hollow back.
(At the same time,just not to leave it unmentioned, there are sites using the term the way LEO does.)
My question, then, should not have been whether the translation of "to arch one's back" in the sense of "to hump one's back" is _correct_.
But in fact,I wonder if the same term at the same time could as well mean an arch in the opposite direction ("hollow back") and how often it is used that way?
My next question then would be how a native speaker might recognise, without pictures, which sort of arch (hump or hollow back) is meant if someone are advised "to arch one's back instead of keeping it straight"?
Thank you for your help - and for your patience.