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Lactic Acid

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 ifwe 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 hydrogen ions (H+). A buildup 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 buildup 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 musclecontraction 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 downthat 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: Disposalof 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 thebody 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

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 musclecells, 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.

Hydrogen ions

The breakdownof glucose or glycogen produces lactate and hydrogen ions - for each lactate molecule, one hydrogen ion is formed. The presence of hydrogen 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...
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