Short legs!
I've also flown the AA5 despite being short (ish) and having short (ish) legs. For me, a lumbar support cushion bought from the local chemist solved the problem completely by shifting me forward in the seat by a couple of inches- and incidentally made the seat much more comfortable, didn't move around, and was easy to carry around to put into different a/c if I encountered similar problems elsewhere. You don't have to have a back problem to find them useful!!