It's not the fact that he's not used to warp that gets him, as it happens. Jim was moving a little too fast down the hallway, trying to catch on to one of the engineers rushing across the ship, and he didn't have enough time to get a good grip on anything before the ship jumped to warp.

Actually Bones might even see him floating right past him while the jump's happening, and when the ship slows to a halt he's tossed against a wall at an unforgiving speed. He groans when he lands on the floor with a loud thud, immediately trying to sit up even though he's holding his midsection with a barely contained wince.

"Shit..."