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.
ENERGY SYSTEMS
The ATP-PCr system
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.
The Glycolytic System
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.
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.
What is Blood Lactate?
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).