During glycolysis (the breakdown of carbohydrate) pyruvate is formed. If the exercise intensity is low then pyruvate goes to the krebs cycle and produces energy using oxygen. If the exercise intensity is high then pyruvate is converted to lactate, along with other byproducts.
This process actually consumes some muscle energy. This inefficient form of energy production occurs because the muscle needs energy at a rate faster than can be supplied by oxidative phosphorylation (burning fat, aerobic) or via the krebs cycle (aerobic glycolysis).
Producing lactate also creates a byproduct called hydrogen ion (H+) which reduces muscle and blood pH (becomes more acidic). This is bad news for the muscle as H+ inhibits muscle contraction. (H+ displaces calcium within a muscle fiber, inhibiting cross-bridge cycling and reducing contraction force).
Contrary to what many may think it is not lactate that causes muscles to stop working or get sore after exercise (people afflicted with McArdle's disease cannot produce lactate and yet they still fatigue with exercise)
Lactate is formed even at rest, although at much lower levels, and is actually a preferred source of energy for the heart and slow-twitch muscle fibers (it gets converted back into ATP, the substance muscles need to contract and stimulates the liver to produce more glucose).
Lactate levels in the blood at rest are typically around 1-2 mmol and can exceed 20mmol during maximal exercise.
Lactate production and removal is a continual process within the muscle. As exercise intensity increases the rate of lactate production increases (our bodies need energy quickly at higher intensities and this pathway produces energy quickly, even at the expense of being inefficient). As intensity continues to rise higher lactate levels exceed the body's clearance capacity (via buffering or being used as a fuel source) and blood lactate levels rise precipitously.
The point where lactate begins to rise quickly is termed LT (Lactate Threshold) or OBLA (Onset of Blood Lactate Accumulation). These terms have replaced the misnomer "anaerobic threshold" because the muscle is never really out of oxygen as this term may imply.
Individual lactate threshold's typically occur around 4 mmol but can range from 3-6mmol depending on the individual.
(OK class pay attention – Extra credit)
*The limitation to exercise above the LT is not the increased levels of blood or muscle lactate but the associated increase in acidosis (H+) and other markers of muscular fatigue*
The physiological changes that occur at this threshold are significant for training the heart and lungs (metabolic acidosis, impaired muscle contraction, hyperventilation, altered oxygen kinetics, etc.).
Endurance training based around this threshold intensity has been shown to improve the absolute workload that can be performed at this threshold (increasing threshold power from 210 watts to 240 watts) as well as the percentage of VO2 max (increasing the threshold from 80% of VO2max to 85%).
This threshold and the percentage of VO2max it occurs are the single biggest predictors of you endurance performance.
To obtain your lactate threshold blood lactate samples are taken (via a finger tip prick) during an incremental exercise test and the results graphed to determine what intensity (HR, watts) a shift in production occurs. A separate VO2max test will further determine what potential an individual has for improvement of this threshold.