To understand what blood lactate is and how it is produced during exercise, it is useful to have a basic understanding of the systems the body uses to produce energy. Whether you’re running a marathon or performing an Olympic lift, skeletal muscle is powered by one important compound; adenosine triphosphate (ATP). The body only stores small amounts of ATP in the muscles so it has to replace and resynthesize this energy compound on an ongoing basis. Understanding how it does this is the key to understanding energy systems.
There are 3 separate energy systems through which the body produces ATP. Describing each of these systems in detail goes beyond the aim of this article. Instead it is intended that the brief outlines provided will assist in describing the role of blood lactate during energy production for exercise, and how this knowledge can be used to help with training for improved endurance performance.
This system produces energy during the first 5-8 seconds of exercise using ATP stored in the muscles and through the breakdown of phosphocreatine (PCr). This system can operate with or without the presence of oxygen but since it doesn’t rely on oxygen to work it is said to be anaerobic. When activity continues beyond this period the body relies on other ways to produce ATP.
This system produces ATP through the breakdown of glucose in a series on enzymatic reactions. The end product of glycolysis is pyruvic acid. This either gets funneled through a process called the Kreb’s cycle (slow glycolysis) or gets converted into lactic acid (fast glycolysis). The fast glycolytic system produces energy more quickly than slow glycolysis but the end product of lactic acid can accumulate and is thought to lead to muscular fatigue. The contribution of the fast glycolytic energy system rapidly increases after the first 10 seconds and activity lasting up to 45 seconds is supplied by energy mainly from this system. Anything longer than this and there is a growing reliance on the Oxidative system.
This is where pyruvic acid from slow glycolysis is converted into a substance called acetyl coenzyme A rather than lactic acid. This substance is then used to produce ATP by funneling it through the Krebs cycle. As it is broken down it produces ATP but also leads to the production of hydrogen and carbon dioxide. This can lead to the blood becoming more acidic. However, when oxygen is present it combines with the hydrogen molecules in series of reactions known as the electron transport chain to form water thus preventing acidification. This chain, which requires the presence oxygen, also leads to the production of ATP. The Krebs cycle and the electron transport chain also metabolise fat for ATP production but this again requires the presence of oxygen so that the fats can be broken down. More ATP can be liberated from the breakdown of fats but because of the increased oxygen demand exercise intensities must be reduced. This is also the most sustainable way of producing ATP.
It is important to remember that these systems are all constantly working to produce energy for all bodily functions and one system is never working exclusively over the others. When it comes to energy production for exercise one system will play a more dominant role (this will be dictated by the type of activity being performed) but all 3 systems will still be working to provide adequate amounts of ATP.
It is through the Glycolytic System that the role and production of blood lactate becomes apparent. Recall the end product of glycolysis is pyruvic acid. When this is converted into lactic acid it quickly dissociates and releases hydrogen ions. The remaining compound then combines with sodium or potassium ions to form a salt called lactate. Far from being a waste product, the formation of lactate allows for the continued metabolism of glucose through glycolysis. As long as the clearance of lactate is matched by its production it becomes an important source of fuel.
Clearance of lactate from the blood can occur either through oxidation within the muscle fibre in which it was produced or it can be transported to other muscle fibres for oxidation. Lactate that is not oxidized in this way diffuses from the exercising muscle into the capillaries and it is transported via the blood to the liver. Lactate can then be converted to pyruvate in the presence of oxygen, which can then be converted into glucose. This glucose can either be metabolized by working muscles (as an additional substrate) or stored in the muscles as glycogen for later use. So lactate should be viewed as a useful form of potential energy. Lactic acid and lactate do not cause fatigue per se.
In fact, it is a common misinterpretation that blood lactate or even lactic acid has a direct negative effect on muscle performance. It is now generally accepted that any decrease in muscle performance associated with blood lactate accumulation is due to an increase in hydrogen ions, leading to an increased acidity of the inter-cellular environment. This acidosis is thought to have an unfavourable effect on muscle contraction, and contributes to a feeling of heavy or ‘jelly’ legs.
The term ‘accumulation’ is therefore the key, as an increased production of hydrogen ions (due to an increase production of lactic acid) will have no detrimental effect if clearance is just as fast. During low intensity exercise blood lactate levels will remain at near resting levels as clearance matches production. As exercise intensity increases there comes a break point where blood lactate levels will start to rise (production starts to exceed clearance). This is often referred to as the lactate threshold (LT). If exercise intensity continues to increase a second and often more obvious increase in lactate accumulation is seen. This is referred to as the lactate turn point (LTP).
The physiological processes discussed above can’t be over ruled when it comes to the limiting factors of endurance performance i.e. you can’t run a marathon once lactate is significantly increasing. An individual’s LT and LTP are therefore powerful predictors of endurance performance. Knowing the exercise intensity that represents these two points can prove to be a valuable tool in assessing a person’s current performance capabilities. In addition it can also help with the construction of an effective training program. With the right kind of training i.e. appropriate volume, intensity and frequency an individual should see a shift in their LT and LTP, whereby the exercise intensity is higher at these two points. This would then be reflected in improved endurance performance as the limiting effects of lactate accumulation don’t occur at the intensity or pace that was observed prior to training. The prescription of training zones to achieve this type of adaptation is based on the heart rate ranges that represent an individual’s original LT and LTP.
Using these heart rate zones, a specific training program can be created to make sure that an appropriate amount of time is spent training at intensities above, below or equivalent to LT and LTP. The main goal is to raise the intensity at which LT and LTP occur and this would in turn be reflected in an ability to work at higher intensities for longer periods of time i.e. the clearance of lactate matches production at a higher intensity and muscle fatigue due to acidosis is delayed. Other benefits of using these specific heart rate zones include making training more specific to a particular event as some events will require more work in certain zones than others. It is also possible to protect glycogen stores and therefore enable a higher training volume whilst avoiding over doing it. Pace judgement can improve as the ability to hold training intensities gets better and doing the right amount of work by following a targeted program can give an athlete confidence and reduce anxiety. Figure 1. Shows what a blood lactate profile may look like before and after a period of appropriate training.
Thanks to the development of blood lactate testing equipment, ascertaining this type of information is relatively easy and can be done outside of a laboratory with a high degree of accuracy. Blood samples can be taken from the ear lobe at various stages during a short sub-maximal incremental test procedure (normally on a treadmill, bike or rowing machine). Instantaneous blood lactate readings can be produced during the test, graphed against intensity and correlated with heart rate, all within a relatively short time frame.
This is not something that is reserved only for the elite population. In fact recreational runners, cyclists and rowers stand to gain more from this type of information as they potentially have more room for improvement. It is for this reason that Matt Roberts Personal Training has added this type of testing to its battery of training focused services. All recreational endurance enthusiasts stand to gain valuable and usable insights into their own physiology with this type of testing and when used in conjunction with a well-structured training program performance is guaranteed to improve.