The expression “lactic acid” is used most commonly by athletes to describe the intense pain felt during exhaustive exercise, especially in events like the 400 metres and 800 metres. When energy is required to perform exercise, it is supplied from the breakdown of Adenosine Triphosphate (ATP). The body has a limited store of about 85 grms of ATP and would use it up very quickly if we did not have ways of resynthesising it. There are three systems that produce energy to resynthesise ATP: ATP-PC, lactic acid and aerobic.

The lactic acid system is capable of releasing energy to resynthesise ATP without the involvement of oxygen and is called anaerobic glycolysis. Glycolysis (breakdown of carbohydrates) results in the formation of pyruvic acid and hydronium ions (H+). The pyruvic acid molecules undergo oxidation in the mitochondrion and the Krebs cycle begins. A build up of H+ will make the muscle cells acidic and interfere with their operation so carrier molecules, called nicotinamide adenine dinucleotide (NAD+), remove the H+. The NAD+ is reduced to NADH that deposit the H+ at the electron transport gate (ETC) in the mitrochondria to be combined with oxygen to form water (H2O).

If there is insufficient oxygen then NADH cannot release the H+ and they build up in the cell. To prevent the rise in acidity pyruvic acid accepts H+ forming lactic acid that then dissociates into lactate and H+. Some of the lactate diffuses into the blood stream and takes some H+ with it as a way of reducing the H+ concentration in the muscle cell. The normal pH of the muscle cell is 7.1 but if the build up of H+ continues and pH is reduced to around 6.5 then muscle contraction may be impaired and the low pH will stimulate the free nerve endings in the muscle resulting in the perception of pain (the burn). This point is often measured as the lactic threshold or anaerobic threshold (AT) or onset of blood lactate accumulation (OBLA).

The process of lactic acid removal takes approximately one hour, but this can be accelerated by undertaking an appropriate cool down that ensures a rapid and continuous supply of oxygen to the muscles.

The normal amount of lactic acid circulating in the blood is about 1 to 2 millimoles/litre of blood. The onset of blood lactate accumulation (OBLA) occurs between 2 and 4 millimoles/litre of blood. In non athletes this point is about 50% to 60% VO2 max and in trained athletes around 70% to 80% VO2 max.

Reference: Disposal of Lactate during and after Strenuous Exercise in Humans, Journal of Applied Physiology, Vol 61(1), pp338-343, 1986

Lactic acid – friend or foe?

Lactic acid (lactate) is not:

* responsible for the burn in the leg muscles when exercising very fast
* responsible for the soreness you experience in the 48 hours following a hard session
* a waste product

Lactate, which is produced by the body all day long, is resynthesized by the liver (Cori Cycle) to form glucose that provides you with more energy. Sounds like a friend to me.
Lactate Shuttle

Some of the lactate we produce is released into the blood stream and used directly as a fuel by heart muscle, and by the liver to produce blood glucose and glycogen (Cori Cycle).

The lactate shuttle involves the following series of events:

* As we exercise pyruvate is formed
* When insufficient oxygen is available to breakdown the pyruvate then lactate is produced
* Lactate enters the surrounding muscle cells, tissue and blood
* The muscle cells and tissues receiving the lactate either breakdown the lactate to fuel (ATP) for immediate use or use it in the creation of glycogen
* The glycogen then remains in the cells until energy is required

65% of lactic acid is converted to carbon dioxide and water, 20% into glycogen, 10% into protein and 5% into glucose.

It has been estimated that about 50% of the lactate produced during intensive exercise is used by muscles to form glycogen which acts as a metabolic fuel to sustain exercise.
Krebs Cycle

The Krebs cycle is a series of reactions which occurs in the mitochondria and results in the formation of ATP. The pyruvic acid molecules from glycolysis undergo oxidation in the mitochondrion to produce acetyl coenzyme A and then the Krebs cycle begins.

Three major events occur during the Krebs cycle. One guanosine triphosphate (GTP) is produced which donates a phosphate group to ADP to form one ATP; three molecules of Nicotinamide adenine dinucleotide (NAD) and one molecule of flavin adenine dinucleotide (FAD) are reduced. Although one molecule of GTP leads to the production of one ATP, the production of the reduced NAD and FAD are far more significant in the cell’s energy generating process because they donate their electrons to an electron transport system that generates large amounts ATP.
Cori Cycle

The Cori cycle refers to the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves via the blood stream to the liver where it it is converted to blood glucose and glycogen.
hydronium ions

The breakdown of glucose or glycogen produces lactate and hydronium ions – for each lactate molecule, one hydrogen ion is formed. The presence of hydronium ions, not lactate, makes the muscle acidic that will eventually halt muscle function. As hydrogen ion concentrations increase the blood and muscle become acidic. This acidic environment will slow down enzyme activity and ultimately the breakdown of glucose itself. Acidic muscles will aggravate associated nerve endings causing pain and increase irritation of the central nervous system. The athlete may become disorientated and feel nauseous.
Aerobic Capacity

Given that high levels of lactate/hydronium ions will be detrimental to performance, one of the key reasons for endurance training is to enable the body to perform at a greater pace with a minimal amount of lactate. This can be done by long steady runs, which will develop the aerobic capacity by means of capillarisation (formation of more small blood vessels, thus enhancing oxygen transport to the muscles) and by creating greater efficiency in the heart and lungs. If the aerobic capacity is greater, it means there will be more oxygen available to the working muscles and this should delay the onset of lactic acid at a given work intensity.
Anaerobic Threshold

Lactic acid starts to accumulate in the muscles once you start operating above your anaerobic threshold. This is normally somewhere between 80% and 90% of your maximum heart rate (MHR) in trained athletes.
What a low Lactate Threshold means

If your lactate threshold (LT) is reached at low exercise intensity, it often means that the “oxidative energy systems” in your muscles are not working very well. If they were performing at a high level, they would use oxygen to break lactate down to carbon dioxide and water, preventing lactate from pouring into the blood. If your LT is low, it may mean that:

* you are not getting enough oxygen inside your muscle cells
* you do not have adequate concentrations of the enzymes necessary to oxidize pyruvate at high rates
* you do not have enough mitochondria in your muscle cells
* your muscles, heart, and other tissues are not very good at extracting lactate from the blood

Improving your Lactate Threshold

The aim is to saturate the muscles in lactic acid in order to educate the body’s buffering mechanism (alkaline) to deal with it more effectively. The accumulation of lactate in working skeletal muscles is associated with fatigue of this system after 50 to 60 seconds of maximal effort. Sessions should comprise of one to five repetitions (depends on the athlete’s ability) with near to full recovery.

Training continuously at about 85 to 90% of your maximum heart rate for 20 to 25 minutes will improve your Lactate Threshold (LT).

A session should be conducted once a week and commence eight weeks before a major competition. This will help the muscle cells retain their alkaline buffering ability. Improving your LT will also improve your tlimvVO2max.