Last night I tested to find my lactic threshold; the point at which you go from training aerobically, to anaerobically. The test is 30 mins max effort on the bike, with your heart rate recorded and averaged out over the last 20 mins. If you go too fast, you won't be able to maintain the effort, so throughout the session you keep 'bouncing' off your anaerobic zone. Armed with this figure, I will be able to train at 90% of my lactate threshold (aerobically) all day long and so increase my cycling speed, but if you don't know 100%, you can't know 90%, or 120% can you?