The lactate threshold (LT) (or lactate inflection point (LIP) or aerobic threshold (AeT)) is the exercise intensity at which lactate (more specifically, lactic acid) starts to accumulate in the blood stream. The reason for the acidification of the blood at high exercise intensities is two-fold: 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.
Accurately measuring the lactate threshold 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.
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).
Aerobic Threshold 
The aerobic threshold (AeT) is sometimes defined as the exercise intensity at which anaerobic energy pathways start to operate and 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. Lactic acid is a by-product of using anaerobic metabolism to create ATP for working muscles.