I get a harder workout riding a fixed gear. Tried the freewheel cog and didn't think it was nearly as hard. There's no "rest" with a fixed gear, either going up, down or flats. There's also the mental aspect of riding a fixed gear. You cannot let your brain lapse one second. There's no room for day-dreaming or thinking about something else. All of the focus is on the bike, the road and the engine or bad things tend to happen...quickly. That mental exercise for me is also a workout. It helps me focus better at everything.