The lactate threshold (LT) is a very important measure of cardiorespiratory endurance as it indicates the ability of the active muscles to sustain aerobic metabolism. The LT is defined as the point in time during increasing exercise intensity when the rate of lactate production exceeds the rate of lactate clearance. With increasing levels of exercise intensity less oxygen is available at the cell for aerobic (aka oxidative) metabolism to take place, as a result non-oxidative, or fast glycolysis occurs. The end stage of fast glycolysis is the formation of lactate rather than pyruvate (under oxidative circumstances pyruvate is sent to the mitochondria for complete catabolism). The formation of lactate in combination with repeated muscle contractions during intense exercise lasting more than 30 seconds also causes an accumulation of hydrogen ions that leads to a more acidic cellular environment (acidosis).
The accumulation of hydrogen ions (H+) in the cellular environment and resultant acidosis is a byproduct of fast glycolysis and this is the primary contributing factor to fatigue during very intense continuous exercise.
It is hypothesized that acidosis causes muscular fatigue by disrupting muscle contraction in two ways:
- Disrupted propagation of central nervous system impulses at the neuromuscular junction, so the t-tubules and sarcoplasmic reticulum do not receive an impulse to release Ca++ and thus initiate muscle contraction. (read “Skeletal Muscle” post if you’re lost)
- Acidosis also disrupts the enzymes responsible for glycolysis, and so energy (ATP) can’t be produced as effectively.
As a result of these mechanisms the athlete is forced to slow down in order to generate ATP. When the athlete slows down more oxygen is available for oxidative glycolysis/ beta-oxidation, lactate metabolism, and lactate clearance. Lactate by itself is actually a very important fuel for your body! Slow twitch muscle fibers catabolize the lactate that is produced by fast twitch fibers in order to generate ATP. So think of lactate as good stuff; it’s constantly being produced, shuttled around, and used give you more energy.
Application to Training
In a nut shell: the harder an athlete can work before reaching the lactate breakpoint the more successful the athlete will be.
The closer the LT is to VO2max the greater the time till the athlete fatigues. Therefore a primary training objective in addition to improving VO2max is to develop a very high LT. In fact the adaptive ability of the muscles and and capillary circulation around the muscles greatly exceeds the adaptive ability to improve VO2max! For example if two athletes with the same VO2max but different LTs compete against each another, then who ever has the higher LT is probably going to win. The higher the LT the longer blood lactate levels stay close to resting levels which means the body is relying more on sustained oxidative metabolic pathways rather than relying on the fast glycolytic pathway. This also means you are using more efficient slow twitch muscle fibers than fast twitch. When those slow twitch fibers start to fatigue then due to the orderly recruitment principle fast twitch fibers start working and more lactate is produced.
On a well-trained blood lactate curve we see a much longer period where the blood lactate levels stay near resting levels throughout increasing intensities. This shows: the efficiency of slow twitch fibers, the body’s ability to clear lactic acid, and the ability sustain aerobic metabolism at higher workloads.
Without frequent measurement of the individual’s LT the body will adapt to the current stimulus and then performance will stagnate. The greatest adaptation in the LT occurs when training intensity is spot on the LT or slightly above it. Training here teaches the body how to deal with greater and greater amounts of lactate + H accumulation by shuttling lactate to other cells for fuel and buffering the acidic environment by way of bicarbonate.
Training near the LT is really strenuous, so should be prescribed for already fit individuals/athletes, and not for untrained people just beginning exercise. Untrained individuals should initially focus on increasing the efficiency of the central circulation: i.e. the ability of the heart, lungs, and circulatory system to deliver oxygen. Moderate continuous exercise will cause improvement in central circulation through an increase in blood volume, cardiac output, hormonal sensitivity, and respiratory control. After improvements have been made (12 weeks) exercise overload can be in the form of LT interval training to develop the ability of the local circulation and active muscles to clear lactate, improve aerobic power, and reduce reliance on the fast glycolytic system for energy.
When assigning LT intervals, durations should begin at a minimum of 8minutes and may increase up to 30min work bouts. The total time spent at LT in a given workout should be an accumulated time of about 30 to 60min. Rest intervals should begin at equal time of work interval (1:1 ratio) and can gradually decrease to cause incomplete recovery, which would create a greater stimulus for adaptation.