I found this article on Joel Friel's old blog. This article is one of three, each article explains 1 of the 3 ways you can improve your fitness in endurance athletics. I'll republish all three articles here on my blog. He hasn't posted on his blog in years, but it's still a great read.
Exercise physiologists generally agree that there are only three things you can improve to become physiologically more fit for endurance sports performance: aerobic capacity, lactate threshold and economy
While aerobic capacity gets a lot of ink in endurance-sport magazines, for the competitive athlete the lactate threshold is what the bulk of the hard training should focus on. Your aerobic capacity isn’t going change a lot if you’ve been training and racing seriously for three or more years. But you may be able to bump your lactate threshold up a lot.
So what is lactate threshold? We need to start with a little biochemistry to understand this measure of intensity.
As your body uses carbohydrate to create energy it creates a by-product inside the working muscle cells called lactic acid. As the intensity of a workout increases this liquid begins to seep out of the muscle cell into the surrounding space and blood stream. In so doing it changes its composition by giving off hydrogen ions. It’s now called lactate. Despite its “bad boy” reputation, lactate is actually a beneficial substance for the body during exercise as it is used to create more energy so that exercise may continue. It’s the hydrogen that is the real bogey man. This is what causes the burning sensation in your muscles and the heavy breathing at high effort levels. Measuring lactate levels in the blood is a convenient way of estimating how much hydrogen is in the body. The more intense the workout, the greater the amount of lactate released into the blood — and the more hydrogen ions interfering with muscle contractions. (By the way, neither lactate or hydrogen ions cause the muscle soreness you may experience the day after a hard workout. That's another of the myths that refuses to die in sport. Some day I will do a post just on such old saws.)
Lactate threshold is sometimes referred to as anaerobic threshold. While sports scientists may argue about the differences between these two terms, for athletes there is little reason for concern. Both are essentially the high intensity at which you begin to “red line.” On a perceived exertion scale of 1 (low) to 10 (high) you redline at about 7 or 8. Whatever your heart rate, power or pace is at this moment is your lactate threshold intensity. The higher this is as a percentage of your aerobic capacity the faster you will race, especially in steady-state events such as triathlons or endurance running races. It’s common with fit athletes for their lactate thresholds to fall in the range of 80 to 85 percent of their aerobic capacities.
Most well-conditioned athletes can sustain this level of intensity for about an hour. Because of this there is a new term created by Hunter Allen and Dr. Andrew Coggan, the authors of Training and Racing With a Power Meter, to describe this intensity – functional threshold. This is the average bike power (functional threshold power – FTPw) or running pace (functional threshold pace – FTPa) you can maintain for one hour. Simple.
If you are using heart rate to determine your training zones, your lactate threshold heart rate (LTHR) is your average heart rate for a one-hour race effort. This is unique to the sport, so your rowing, cross-country skiing, swimming, cycling and running LTHRs are likely to be different. And therefore your heart rate zones will also be unique to each sport.
The body has two ways of improving your lactate threshold as a result of training. It can come to better tolerate the acid and it can also become more effective at removing the acid. As with all aspects of fitness, the way to train your body to tolerate and remove hydrogen ions is by training at your lactate threshold. This, then, is the best marker of training intensity. That’s why I base heart rate zones on it rather than on maximum heart rate.