The anaerobic threshold (AnT) represents a pivotal concept in exercise physiology, marking the intensity of exercise beyond which lactic acid begins to accumulate in the blood. This threshold signifies a transition in the body's energy production, moving from predominantly aerobic pathways to a greater reliance on anaerobic metabolism. Dr. Karlman Wasserman, in 1964, defined this threshold as the exercise intensity where lactic acid accumulation in the blood becomes
significant. He recognized the profound importance of this finding, understanding that lactic acid buildup could indicate a health risk due to insufficient oxygen supply to the working muscles. The anaerobic threshold is now widely used in both diagnosing diseases and optimizing athletic training globally.
At the anaerobic threshold, exercise intensity reaches a point where the blood lactate concentration is not only linearly related to exercise intensity but also increases with both intensity and duration. The specific blood lactate concentration observed at this point is referred to as the "maximum steady-state lactate concentration" (MLSS). This threshold typically occurs when an individual's heart rate is between 65% and 85% of their maximum heart rate. The onset of anaerobic energy pathways at this intensity is also associated with blood lactate concentrations reaching approximately 2 mmol/liter, compared to a resting level of around 1 mmol/liter.
Measuring and Interpreting the Anaerobic Threshold
Measuring the anaerobic threshold is crucial for understanding an individual's physiological response to exercise. One common method involves analyzing blood samples to determine lactate concentration. However, the accuracy of these measurements can be influenced by several factors. An individual's health status, including their glycogen stores and the ambient temperature prior to exercise, can affect the blood lactate response. Furthermore, the lactate concentration measured can vary depending on the sampling site, potential sweat contamination, and the precision of the lactate analyzer used. It is essential to consider these variables to ensure the reliability of the test results.
Another approach to determining the anaerobic threshold involves analyzing ventilatory and cardiovascular gas exchange responses during exercise testing. This method, described by Dr. Wasserman, focuses on changes in the composition of expired air. The ventilatory threshold (VT1) is a related concept in kinesiology, referring to the point during exercise where the volume of air breathed out increases exponentially faster than oxygen consumption. VT1 is thought to reflect the anaerobic threshold, as it indicates the point where oxygen supply to muscles no longer meets demand, leading to lactate accumulation and an increased need to exhale CO2 produced during the conversion of lactic acid to lactate.
Training and the Anaerobic Threshold
Training plays a significant role in modifying the anaerobic threshold. Regular endurance exercise leads to adaptations in skeletal muscle that can raise this threshold. These adaptations are partly mediated by the activation of the protein receptor PGC-1α, which alters the isoenzyme composition of the lactate dehydrogenase (LDH) complex. Specifically, it decreases the activity of lactate dehydrogenase A (LDHA) and increases the activity of lactate dehydrogenase B (LDHB), influencing how lactate is processed.
While aerobic training primarily increases the lactate threshold, anaerobic training focuses on enhancing tolerance to the effects of lactic acid over time. This allows muscles to continue working effectively even in the presence of increased lactic acid. Exercising at or slightly above the anaerobic threshold is a highly effective strategy for improving lactic acid tolerance, which in turn boosts an individual's capacity for sustained high-intensity effort. The most rapid gains in aerobic conditioning are often made when training close to the anaerobic threshold, as this is the intensity where the heart and lungs struggle to provide adequate oxygen, leading to an oxygen debt and the onset of anaerobic exercise.
