Lactic acid build up largely depends on heart rate zones, and the amount of carbs left stored for immediate use, in threshold zones the body cannot break down fats enough to be turned into carbs, as the fats are being broken down they are in an acidic state which then gets transported to the muscles still as acid, as the muscles are needing carbs faster than they can be produced, you will feel lactic acid build up as you are riding (burning feeling in your legs) you can burn off the acid pretty quickly by bringing the heartrate down and spinning, not really at a fast pace but easy, you can still get lactic acid build up by spinning if your heartrate is too high and burning more carbs than you body can produce. it is a very complex subject lactic acid and muscle recovery, oxygen in the blood etc and it's effects to explain without writing an essay,