Lactic Acid and Lactate Threshold
A by-product of the anaerobic energy production system is lactic acid. Lactic acid is often viewed as an evil demon, but in fact it is an energy source for the body. When given enough time, the body can process and use lactic acid to produce ATP. Lactate (a salt of lactic acid) is present in the blood at rest. Even while you are sitting and reading this column, there are low levels of lactate circulating in your blood stream.
At low levels, lactic acid is not a problem. As you continue to increase your workout intensity, your body increases energy production, relying more heavily on anaerobic metabolism. More reliance on anaerobic metabolism means the lactate level in your blood begins to increase. When your body can no longer process lactic acid fast enough, lactate begins to accumulate at an increasing rate in the blood, the condition is called "onset of blood lactate accumulation" (OBLA) or "lactate threshold." This accumulation is closely correlated with heart rate and ventilatory rate. Athletes can often tell when they have reached lactate threshold because their breathing becomes labored and shortly after that they begin to feel "burning" in their muscles.
If athletes exceed lactate threshold pace by a large margin, they can only sustain the increased pace for a few minutes before the discomfort forces them to slow down. The margin by which lactate threshold is exceeded is inversely proportional to the time the athlete is able to sustain that pace. In other words, if an athlete's lactate threshold heart rate is 162 and heart rate is pushed to 172, he or she will be able to hold that pace for a shorter period of time than if working at a heart rate of 164.
Lactate threshold can be understood as the pace, and correlating average heart rate, that an athlete can sustain for approximately one hour while participating in a single sport. For example, the lactate threshold for a highly fit cyclist is approximately the pace and average heart rate that the athlete can hold for a 40km-time trial on the bicycle. Research has found that lactate threshold heart rate varies depending on the particular sport and the athlete's sport history.
Lactate Threshold Varies Depending on Fitness
Lactate threshold typically occurs at 55 to 65 percent of VO2 max (a quantitative measure of an individual's ability to transfer energy aerobically) in healthy, untrained people. In highly trained endurance athletes, lactate threshold is often greater than 80 percent of VO2 max. Lactate threshold is trainable, and that's good news. In other words, you can train your body to process lactate at higher percentages of VO2 max, which means increased levels of speed before the onset of discomfort forces an end to the effort.
Studies have shown lactate threshold to be a reliable predictor for endurance race performance. VO2 max is not nearly as reliable. So, if you have been tested for VO2 max and your numbers weren't stellar, do not panic.
It is important to note that the training zones are approximations. If you were to test blood lactate levels daily for a period of time, you would find that a heart rate of 162 would produce some variation in the levels of lacate. The more experience you gain as an athlete, the more tuned in you will become to your personal exercise intensity levels.
Use Lactate Threshold to Improve
With your training zones in hand, based on laboratory or field tests to estimate lactate threshold, you can then use a training plan to improve your performance.
Bernhardt, G., Training Plans for Multisport Athletes, CO: VeloPress, 2007.