Before beginning the discussion of anaerobic threshold, there are a couple common misconceptions I would like to address-

1) When anaerobic metabolic pathways are active, no oxygen is present.

TRUTH: The body is never relying on anaerobic metabolism alone. Aerobic and anaerobic pathways are active concurrently; the balance merely shifts from one pathway to another when exercise intensity changes. Therefore, just because anaerobic pathways are preferentially activated does not mean cells do not have any oxygen available.

2) Lactic acid is what builds up in the bloodstream during intense exercise.

TRUTH: Upon production, lactic acid is immediately dissociated into lactate plus a hydrogen ion. Lactate build-up occurs in the bloodstream and may be measured in order to determine the lactate threshold.

At low to moderate exercise intensities, the body relies primarily on aerobic metabolism. When exercise intensity increases, the body shifts to utilization of anaerobic metabolism (however, aerobic metabolism is still occurring, just at lower levels) (1). The increased reliance on anaerobic pathways results in the build-up of lactate in the bloodstream. Also, during high intensity exercise, there is a sudden increase in carbon dioxide excretion. Lactate threshold is determined by the point in exercise where lactate clearance cannot keep up with lactate production, and thus lactate begins to accumulate in the bloodstream. Anaerobic threshold is determined by the exercise intensity where lactate increases above resting levels, and ventilation and excretion of carbon dioxide increase disproportionately to uptake of oxygen. In most cases, lactate threshold and anaerobic threshold occur at the same point in exercise. Because lactate threshold requires sampling of blood, anaerobic threshold is often used as a non-invasive way to estimate lactate threshold (2).

During anaerobic threshold testing, the volume of air expired, as well as the percentage of oxygen and carbon dioxide in the expired air, is measured. The ratio of the volume of air expired (V_E) to the volume of carbon dioxide in the expired air (VCO₂), and the ratio of V_E to oxygen uptake (VO₂) are used to determine the anaerobic threshold. Anaerobic threshold occurs at the point where V_E/VO₂ increases without an increase in V_E/VCO₂ (2).

In terms of athletic capacity, just how important is anaerobic threshold? In the past, VO₂max was utilized as the primary indicator of exercise capacity. We’ve all heard of athletes, such as Lance Armstrong, who have a VO₂max that is off the charts. However, not all elite athletes have such high VO₂max values. Recently, evidence has pointed to the importance of lactate/anaerobic threshold in predicting endurance capacity (2, 3). So don’t get discouraged if your VO₂max is not as high as you would hope, there is more to the story!

References:

(1) Owles WH. Alterations in the lactic acid content of the blood as a result of light exercise and associated changes in the CO2 combining power of the blood and in the alveolar CO2 pressure. Journal of Physiology. 69:214-237, 1930.

(2) Wilmore JH and Costill DL. Physiology of Sport and Exercise: 3^rd Edition. Champaign, IL: Human Kinetics. 2005.

(3) Karlsson J, and Jacons I. Onset of blood lactate accumulation during muscular exercise. I. Theoretical considerations. Int J Sports Med. 3: 190-201, 1982.