Warmaster Horus
Explorer
The game is about story telling, not faithfully recreating physics and kinesiology. Situations like these are what a DM is for and they don't need to be consistent about them unless they want to, because every situation is different. The player can ask, and the DM might say, "Prone", in one case, "Standing" in another, and "If you make your Athletics check by more than 5, the PC can be standing or prone", or whatever.