|This article needs additional citations for verification. (January 2014)|
The lactate threshold (LT) or lactate inflection point (LIP), is the exercise intensity at which the blood concentration of lactate and/or lactic acid begins to exponentially increase. The reason for the increased lactate concentration in the blood at high exercise intensities muscle cells release lactate into the blood to be converted back into glucose by the liver (see the Cori Cycle): the high rates of ATP hydrolysis in the muscle release hydrogen ions, as they are co-transported out of the muscle into the blood via the MCT— monocarboxylate transporter, and also bicarbonate stores in the blood begin to be used up. This happens when lactate is produced faster than it can be removed (metabolized) in the muscle. When exercising at or below the LT, any lactate produced by the muscles is removed by the body without it building up.
With a higher exercise intensity the lactate level in the blood reaches the anaerobic threshold (AT), or the onset of blood lactate accumulation (OBLA).
The lactate threshold is a useful measure for deciding exercise intensity for training and racing in endurance sports (e.g. long distance running, cycling, rowing, swimming and cross country skiing), but varies between individuals and can be increased with training. Interval training takes advantage of the body being able to temporarily exceed the lactate threshold, and then recover (reduce blood-lactate) while operating below the threshold and while still doing physical activity. Fartlek and interval training are similar, the main difference being the structure of the exercise. Interval training can take the form of many different types of exercise and should closely replicate the movements found in the sport.
Measuring lactate threshold
Accurately measuring the lactate blood concentration involves taking blood samples (normally a pinprick to the finger, earlobe or thumb) during a ramp test where the exercise intensity is progressively increased. Measuring the threshold can also be performed non-invasively using gas-exchange (Respiratory quotient) methods, which requires a metabolic cart to measure air inspired and expired. Another way to measure lactate levels non-invasively is using Near-infrared spectroscopy. There are wearable NIRS sensors for professionals, so they can monitor their lactate levels real-time.
Although the lactate threshold is defined as the point when lactic acid starts to accumulate, some testers approximate this by using the point at which lactate reaches a concentration of 4 mM (at rest it is around 1 mM).
Lactate Measurement of Aerobic and Anaerobic Thresholds
The aerobic threshold (AeT or AerT) is sometimes defined as the exercise intensity at which blood lactate concentrations rise above resting levels. Anaerobic threshold (AnT) is sometimes defined equivalently to the lactate threshold(LT); as the exercise intensity beyond which blood lactate concentration is no longer linearly related to exercise intensity, but increases with both exercise intensity and duration. The blood lactate concentration at the anaerobic threshold is called the "maximum steady-state lactate concentration" (MLSS).
Aet is the exercise intensity at which aerobic energy pathways start to operate. Some have suggested this is where blood lactate reaches a concentration of 2 mmol/litre (at rest it is around 1). This tends to be at a heart rate of approximately 20-40 bpm less than the anaerobic threshold and correlates with about 65% of the maximum heart rate. As its name suggests, the anaerobic energy system does not utilize oxygen to create Adenosine triphosphate (ATP) and uses glycogen/glucose instead.
- Matthew L. Goodwin, M.A., James E. Harris, M.Ed., Andrés Hernández, M.A., and L. Bruce Gladden, Ph.D. (Jul 2007). "Blood Lactate Measurements and Analysis during Exercise: A Guide for Clinicians". J Diabetes Sci Technol. 1 (4): 558–569. PMC 2769631. PMID 19885119.
- Moran, P, Prichard, JG, Ansley, L, and Howatson, G (Feb 2012). "The influence of blood lactate sample site on exercise prescription". J Strength Cond Res 26 (2): 563–567. doi:10.1519/JSC.0b013e318225f395. PMID 22240552.
- Mann T, Lamberts RP, Lambert MI (Jul 2013). "Methods of prescribing relative exercise intensity: physiological and practical considerations". Sports Med 43 (7): 613–625. doi:10.1007/s40279-013-0045-x. PMID 23620244.