One of the Xsyon character stats is stamina. In the image below it is the top bar second from the left. It regenerates over time yet even my character with 13 skill points in running will eventually run out of stamina so that he can no longer move for about 5 seconds (sometimes one gets an out of breath animation and other times the character just freezes). Going up and down hills does not seem to affect the rate of stamina drain yet.
Swimming reduces stamina the fastest. The image below shows the distance a character cannot swim across without having all his stamina drained. The character can get about 7/8 across this distance before running out of stamina at which time it freezes in the water although I suspect he will drown once the death features are implemented in Xsyon.