Metabolic Testing Part II: Anaerobic Threshold

A few weeks ago, Staff Coach Joe Ferraro shared his insights on VO2max here in this blog. Today he’ll continue what he started and share his insights on anaerobic threshold, another quality that we test for in our metabolic panel. Once again, get ready for some #science, #knowledge, and #application!

Athletic performance in various competitive sports is influenced largely by the integrated status of a multitude of physiological systems of the athlete, such as the capacity for physiological responses to meet the challenges of the situation, regardless of technique, tactics, and skill. In our previous metabolic testing blog, we discussed the importance of VO2max testing as it relates to optimizing performance.

Traditionally, the most important physiological factor for high performance in endurance events was thought to be a high aerobic capacity or VO2 max. An elite marathoner elicits a VO2 max greater than 80 ml/kg/min. Similarly, VO2 max is an important measurement for middle and long distance runners, road cyclists, long distance swimmers (70–80 ml/kg/min) and team sports players (60–70 ml/kg/min). In long-term endurance events, oxidative phosphorylation plays the dominant role in exercise metabolism and thus maximum oxygen uptake becomes one of the major determining factors in high performance sports.

rio-olympics-800m
The 800-meter run is considered a middle distance event.

VO2 max may be the most physiologically significant and most commonly measured parameter in the physiological assessment of athletes. A high aerobic output is an advantage in endurance sports, but what about the ability to sustain a high fraction of VO2 max for a longer period of time? This is actually way more important, and the fraction of VO2max that one is able to sustain is often referred to as anaerobic threshold. the often misunderstood and improperly defined Anaerobic Threshold.

What is Anaerobic Threshold?

The term anaerobic threshold was introduced in the 1960’s based on the concept that during high-intensity levels of exercise, low levels of oxygen are present in the muscles (Roberts & Robergs 1997). At this point, to allow for exercise to continue, there needed to be a shift in energy supply from the aerobic energy system (oxidative phosphorylation) to anaerobic energy systems (glycolysis and phosphagen system).

However, there are many exercise physiologists who strongly object to using the term anaerobic threshold, believing it to be misleading. Thus, AT is often misunderstood and improperly defined. The main argument against using the term anaerobic threshold is that it suggests oxygen supply to muscles is limited at specific exercise intensities. However, there is no evidence that indicates muscles become deprived of oxygen, even at maximal exercise intensities (Brooks 1985). Additionally, anaerobic threshold suggests at this point in exercise intensity, metabolism shifts completely from aerobic to anaerobic energy systems. This interpretation is an overly simplistic view of exercise metabolism as anaerobic energy systems (glycolysis and the phosphagen system) do not take over the task of ATP regeneration completely at higher intensities of exercise, but rather enhance and add to the energy supply provided from oxidative phosphorylation. (Robergs, Ghiasvand, Parker, 2004).

More recent research of physiological parameters helps to define “Anaerobic Threshold” in more specific terms: Lactate Threshold & Ventilatory Threshold.

A quick Google search yields the definite of AT as the following: the exercise intensity at which the concentration of lactate in blood begins to increase exponentially. Concepts of lactate threshold and ventilatory threshold were introduced in order to define the point when metabolic acidosis (caused by lactate/lactic acid) and the associated changes in gas exchange in the lungs occur during exercise, respectively.

tour-de-franceStately simply, during incremental exercise, there is a nonlinear steep increase in ventilation (the ventilatory threshold), a non linear increase in blood lactate concentration (known as the lactate threshold), a non linear increase in CO2 production, an increase in end tidal oxygen, an increase in CO2 production, an increased arterial lactate level of 4 mM/L, known as onset of blood lactate accumulation (OBLA), and an abrupt increase of FEO2 (expired O2 fraction).

All these points are collectively labeled as Anaerobic Threshold (AT). There is evidence to support that ventilatory anaerobic threshold is directly related to and also caused by blood lactate threshold.

Importance of Anaerobic Threshold Testing: A Quick Physiology Lesson

During intense exercise, lactate production increases 5-7 times higher than that of resting levels and the release of hydrogen ions (H+) associated with lactate can cause a reduction in contractile muscle pH, resulting in metabolic acidosis. This accumulation of H+, not only from lactate but also from the breakdown of ATP may interfere with muscle contraction at enzymatic sites by competing with Calcium (Ca++) for Troponin C. Hydrogen ions (H+) may also inhibit calcium release and reuptake from sarcoplasmic reticulum involved in muscle contraction. The result is a decrease muscle contraction capacity, and therefore a decrease in peak force and muscle shortening velocity.

Furthermore, lactate is not the waste product it was once made out to be and it is in fact, an important precursor for allowing anaerobic metabolism to continue. Lactate can be exported to the blood for energy purposes in pretty much every organ in the body. However, this process takes several minutes while lactate is produced continuously during exercise. Elite athletes are very efficient and export less lactate to the blood, as they utilize it for immediate energy in higher amounts right in the lactate-producing muscle. This is very advantageous as it allows active muscles to remove H+ at a faster rate, recycling lactate for extra ATP within the muscle. During exercise, lactate is mainly produced in fast twitch muscle fibers and is taken up by nearby slow twitch muscle fibers.

anaerobic-threshold-before-after
Notice how anaerobic threshold is a higher percentage of VO2max after training, and thus there is a delayed spike in blood lactate concentration.

The important point here is that lactate threshold is exceeded as a result of an increased reliance on carbohydrate metabolism (glycolysis) during high intensity efforts. This level of intensity cannot be sustained because of the accumulation of H+ ions and decreased contractile ability of the muscles. Elite athletes possess a higher anaerobic threshold not only because of their increased ability to utilize lactate as a fuel source, and therefore thwart the accumulation of H+, but also because they have a larger capacity to perform under aerobic conditions in which fat metabolism is the primary source of energy and carbohydrate stores are preserved.

 

Training to improve the lactate threshold

Completing an anaerobic threshold test presents an athlete with proper training zones to increase metabolic efficiency, a key physiological variable of athletic performance. The #1 predictor of performance for an athlete during an endurance event has been found to be the velocity that s/he is able to maintain during the event.

There are a number of training methods and intensities that have been shown to improve the percentage of VO2max at which anaerobic threshold occurs. I like to see all of these strategies employed

Low–moderate intensity aerobic based training – This is one of the most important training intensities for the development of the lactate threshold. It involves training at an intensity of 70-80% and should consist of more than 50% of your total aerobic training volume. Training at this intensity has been shown be very vital for the development of slow twitch muscle fibers, mitochondrial size and density, muscle capillarization and aerobic energy production.

Tempo training – This involves performing 10-20 minute splits at an intensity that corresponds with, or is slightly below your anaerobic threshold. Training this way improves the ability to sustain intense workloads over prolonged periods and may increase the percentage of VO2max at which the threshold occurs by improving the muscle cells ability to utilize lactate during aerobic metabolism. An example anaerobic threshold interval session would be 2 sets of 10 minute intervals at an appropriate speed or power output (determined through testing to be at or slightly below AT), with a 5-minute active recovery between the 10-minute intervals. A training program should include 10-15% of volume through anaerobic threshold training this way.

High intensity interval training – This includes intervals above the anaerobic threshold and should consist of 3-4 minute bouts at around 95-100% of VO2max, with 1-1.5 minute active recoveries. Training at this intensity can lead to increases in the velocity or power output at VO2max and AT. This type of training is very strenuous however, and places the greatest risk of overtraining and injury. High intensity interval training should typically make up no more than 5-10% of your training volume.

When it comes to endurance performance, remember that often times, it’s not your maximum aerobic capacity that matters, but your ability to sustain a high percentage of that capacity for a long period of time. This is what anaerobic threshold is about. The higher it is, the more likely you are to be in a neutral, “cruise-control” zone as far as your metabolic profile is concerned. If you’re interested in finding out your anaerobic threshold, be sure to sign up here! I want to know my anaerobic threshold.

Happy training!

Sources:
Anderson, G.S., & Rhodes, E.C. 1989. A review of blood lactate and ventilatory methods of detecting transition threshold. Sports Medicine, 8 (1), 43-55.

Brooks, G.A. 1985. Anaerobic threshold: review of the concept and directions for future research. Medicine and Science in Sport and Exercise, 17 (1), 22-34.

McArdle, W.D., Katch, F.I., & Katch, V.L. 1996. Exercise Physiology: Energy, Nutrition, and Human Performance. Baltimore, MD: Williams & Wilkins.

Robergs, R.A., & Roberts, S. 1997. Exercise Physiology: Exercise, performance, and clinical applications. St Louis, MO: Mosby.

Robergs, R.A., Ghiasvand, F., & Parker, D. (2004). Biochemistry of exercise-induced metabolic acidosis. American Journal of Physiology: Regulatory, Integrative and Comparative Physiology. 287