The most difficult thing about shooting at a gym is having mirrors on every wall! I spent most of the time while setting up trying to get everything into a position where the flashes wouldn't be visible - often only to find that suddenly I was in a picture (try shooting in a spot that has mirrors on three walls). We moved around until we finally found a spot in the cross-fit section where the bench and dumbbell don't really belong and then managed to set up all the equipment so that it would work.
One test shot with the water to make sure that our model knew how hard to press the bottle and off we went! Of course, after having messed around for such a long time and having kept the area clear until the actual shot could be taken, someone went and started training in the background, completely oblivious to our activities - see below. The shot was just right though, so we decided we would cut him out in post. I had a test shot which was used to overlay and Robert's your father's brother.