:up: But if you don't sit up straight in a chair you slump. If something's wrong (drugged/dead/injured/asleep...) you can also 'slump forward'. But if you just slump (no preposition) you simply fail to sit upright. 'No wonder back pain is so common; after a day slumped in front of a computer we drive home to spend the evening slumped in front of the TV.'
{Hmm, that's interesting, two 's _ _ _ p' words both referring to bent backs (and as a mnemonic, you need lumbar support to stop you slumping.} [Nurse, he's out of his bed again. ;-)]
b
PS AND (more vocab) if you don't stand up straight just because you're lazy or uninterested you 'slouch'.