Very early on in most runner's careers they hear how the build up of lactic acid causes them to run slower and feel sore. Well, it turns out that lactic acid isn't produced in exercise and that lactate, which is produced during exercise, doesn't cause any pain or soreness in our muscles (Noakes). In fact, lactate might be a good thing for us - we just don't know. That should show you how insanely complex our bodies are when you consider that in 2008 we don't entirely understand what happens when we propell ourselves forward.
However, both Daniels and Pfitzinger talk about lactate threshold as if it still means something. Why? Well, for one we seem to be able to predict race times most reliably based on the pace at which lactate starts accumilating more rapidly in the blood. Lactate accumilation is correlated with the build-up of hydrogen ions which lowers pH (acidosis). Hydrogen ions can also block the uptake of calcium - which restricts muscle contraction.
So, what does all this mean? Running around the "lactate threshold" (LT) point (usually 75-80% of V02 max or 15k-20k race pace) does seem to improve running ability. And runners with lactate thresholds at a higher percentage of V02 max race faster. The most popular LT workout is to run at LT pace for 20-40 minutes. Another LT workout is to run mile repeats at LT pace with 1-2 minutes rest.
So, a week from today when I run my first LT workout - I'll try to not confuse myself with the details and just run controlled and strong - because that's what I want to do on 4/20.