Lactate Threshold in Running

During exercise, when glucose (sugar) is burned by the muscle cells for energy, two results can be obtained, depending if enough oxygen is available to the cells. If sufficient oxygen is available, the glucose is burned until only carbon dioxide and water remain, and those substances are expelled by the lungs. This exercise is an aerobic exercise. If sufficient oxygen is not available due to the intensity of the workout, lactate is produced and is absorbed by the blood. This exercise is an anaerobic exercise. The point at which the exercise changes from aerobic to anaerobic exercise is the lactate threshold.

New runners should avoid anaerobic running because they are still developing a base of endurance. Long slow distance (see left-sidebar) and low-level forms of speed (see left sidebar) are done as aerobic exercises. Other runners, especially older ones, who want to avoid intense training should also not run anaerobically. It is thus important that runners be able to recognize when they go from aerobic to anaerobic training. I asked one of my running friends, Randy, to explain how a runner can recognize that he or she has gone anaerobic, and Randy explained it this way.

There are 2 ways a runner can perceive the impact of running beyond the threshold pace. First, you will incur an oxygen debt. This will manifest itself in a feeling of shortness of breath; that your lungs are not large enough to suck in all the air you want. Second, as lactate accumulates you will feel a 'burning' sensation in your muscles.
In general a highly trained, elite, runner will have a marathon pace just under their lactate threshold; for less conditioned/slower runners it might be closer to their 10k pace (or about the fastest pace they can maintain for an 1 hour).

For many runners, the anaerobic threshold is around 85-90% of their maximum heart rate (see left sidebar). However, running anaerobically can help runners go faster, because lactate is an excellent source of energy if one can learn to burn it as fuel. This use of lactate as fuel can be developed by running anaerobically for relatively short periods of time followed by slow, aerobic recovery periods. This method of anaerobic training is briefly explained by Dr. Gabe Mirkin.

A good article on lactate threshold is in Wikipedia.