The term anaerobic threshold is widely used in a number of training environments. Coaches, athletes, trainers, and the like commonly refer to the anaerobic threshold as a focal point to determine the intensity of training. Consequently, much focus has been placed upon accurate measurement of the anaerobic threshold and its interpretation for use in training programs. But first we need to clearly understand what the term really means.
Several terms are used synonymously to denote the anaerobic threshold—anaerobic threshold, lactate threshold, and ventilatory threshold. They basically all refer to the same thing, which is the level of exercise where a steady state blood lactate concentration is no longer observed, and instead, it accumulates steadily. The general term used is anaerobic threshold (AT), and this implies either lactate threshold or ventilatory threshold; we simply use one term or other depending on how we measure it, i.e. using blood or expired gases. The lactate threshold occurs for several reasons, and it is truly a highly integrated time point.
When we start to exercise slowly, the forces of muscle contraction are met primarily by slow-twitch muscle fiber which preferentially burns fat. As intensity increases, higher forces must be produced. These higher forces eventually are met by recruiting more fast-twitch fiber, which prefer to burn carbohydrate. A by-product of carbohydrate metabolism is lactic acid. As the lactic acid accumulates, it increases the acidity in the muscle and eventually starts to inhibit the contractile abilities of the muscle, and you have to slow down. Therefore, in training, we need to specifically address the issue of lactate clearance and tolerance because you essentially compete at higher levels and must learn to compete in this zone.
It is interesting that a common perception of the AT is that time spent above it is contentious in some way, yet it is of utmost importance if you want to be competitive. Simply put, if you want to run faster you have to run fast. The mistake athletes make is that they tend to start training above AT before they are ready. By that I mean that they must have an appropriate aerobic base, otherwise they simply can’t recover from higher intensity workouts or won’t last that long. AT must also be considered in tandem with one’s VO2 max. Many athletes are fit with high VO2 maxes, yet don’t perform accordingly. A like explanation is that the AT is low, and it is the percent of max VO2 at which AT occurs that often dictates performance. Thus, for some athletes, focusing on improving AT is way more important than trying to improve VO2 max. This is where training programs that are individually designed are important as opposed to a generic computer-generated program based on a single VO2 score and age.
In sports of longer distance such as the marathon, cycling, and triathlons, knowledge of AT becomes even more important since there is a relationship between being above or below AT and which fuel source is used—carbohydrates or fats. This is important because our body can only store so many carbohydrate calories, and these become the limiting factor in endurance sports. This is also why nutrition strategies during longer events can become a matter of “complete or collapse.” Accurately knowing your AT can allow you to calculate energy expenditure specifics of carbohydrates and fats at any intensity, and then you can create your replenishment strategy more accurately.
Anyway, back to the original discussion of the AT. At a specific intensity, lactate accumulation occurs at a non-linear rate that ultimately compromises muscle contraction. Thus, the fiber distribution of an individual also can affect the anaerobic threshold. Since increasing levels of lactate indicate higher levels of carbon dioxide, ventilatory rates increase. Thus we also see a non-linear rise in ventilation (hence the term ventilatory threshold). The intensity of exercise is the primary determinant of lactate accumulation, and the higher the intensity the greater the accumulation. Thus, events such as the 400 meter and 800 meter (runs) produce much higher lactate concentrations than do the 3000 meter or 10,000 meter. You should note that the body always produces lactate, and resting levels in trained athletes are typically between 1.5 and 2.0 mmol/L.
The anaerobic threshold varies widely between individuals and between sports. It can also vary substantially within individuals through training. Anaerobic threshold can occur anywhere from 50% to >92% of VO2 max. Untrained individuals typically have low anaerobic thresholds and well-trained individuals have higher anaerobic thresholds, so it can be improved. This typically requires faster speeds of movement regardless of whether you row, swim, bike, or whatever. Intervals are a simple and effective way to do this. Progressive intervals, whereby the distance covered is shorter but the intensity is higher, can help accomplish this. Longer, high-intensity intervals early in the session will initially increase lactate levels, forcing the muscle to work in a more acidic environment and stimulating greater biochemical adaptation. But pay attention to my earlier comments of making sure you are fit enough to do them.
Note: This next session assumes you accurately know your AT.
So, how do you know if your anaerobic threshold workouts are effective? First, your ability to perform in what was previously the anaerobic zone is now more comfortable, and you should be able to sustain slightly higher workloads (speeds) for a longer time. Of course, this relies on you knowing where your anaerobic threshold lies. This is a sensitive test with abundant opportunity for error. It really ought to be performed in a laboratory environment. To do this, typically, four of five workloads, each lasting about three to six minutes, are chosen that ultimately require the athlete to finish at >90% of max capacity. Longer stages are chosen to allow circulating lactate levels to stabilize, and a blood sample is taken at the end of each stage. Anaerobic threshold is generally assumed to be where blood lactate concentration reaches 4mmol/L, but this does vary.
In our lab we plot lactate and ventilation against heart rate and time, to give us a better method of determining heart rate at anaerobic threshold. We also look at carbon dioxide production in relation to oxygen consumption. Simple field tests are available and just require the coach or athlete to be creative. The key is to create a protocol whereby small work increments of four to five minutes are performed. The athlete can then assess breathing or comfort and, importantly, heart rate. When you go above the AT these variables will not plateau, but instead climb slowly until you fatigue.
The reason anaerobic threshold is assumed to be of importance is that an athlete can only spend a limited amount of time exercising above the anaerobic threshold. Therefore, increasing the anaerobic threshold allows the athlete to exercise at a higher intensity for longer periods. Additionally, as I mentioned earlier, glycogen becomes the primary fuel source above the anaerobic threshold, and there is a limited supply of glycogen. Glycogen depletion is arguably as big a problem as high lactate above the anaerobic threshold, especially in longer distances.
One should note that the value of anaerobic threshold in training is not universally accepted. Measurement errors, individual variability, diet, and more, can all affect the outcome. Also, quite often localized muscle action influences lactate concentrations that do not represent total body status. Then there is the question of using the information correctly. Consider this little tale in which I have omitted identification for obvious reasons. In 1996, a U.S. team was lactate profiled prior to the Atlanta Olympics. They were coming off a European tour where they had displayed their worst performances in three years. USOC test results indicated improved fitness from previous tests yet performance of the team was worse than the previous four Olympics. In 1994, the team was World Champion; in 1998 World Championship bronze medalists, and in 1996, fifth out of six in Atlanta, where fitness results indicated better conditioning. The shrewd reader could argue that other teams had made greater improvements in recent years and that although the U.S. team was making progress, others had made more. Or was testing data used incorrectly? I leave you to ponder.
Regardless, knowledge of the anaerobic threshold will ensure higher intensity training that will allow you some measure of change in cardiovascular response to a given workload. This simple approach alone will reinforce whether or not your performance times are improving. In all cases your success will be dependent upon your ability to monitor and evaluate your responses to a given workload, so good record keeping is a must. And perhaps most importantly, the data must be individualized.
Declan Connolly is a professor of exercise physiology and kinesiology at the University of Vermont, a fellow of the American College of Sports Medicine, a certified strength and conditioning specialist, past president of the New England American College of Sports Medicine, and consultant to the NHL, NFL, IOC, and many others. Check out www.vermontfit.com for more information, training tips, articles, etc.
