Lactic Acid

If you have ever felt a deep, painful burning sensation in the muscles during periods of intense exercise, that feeling was likely the by-product of lactic acid accumulation. Lactic is a chemical compound produced by the body throughout the day and is resynthesized by the liver (Cori Cycle) to form glucose that provides you with more energy. During periods of intense exercises, lactic is produced faster than the ability of the bodies' tissues to remove it. As a result, lactic concentration increases and a crippling pain is felt. However, the presence of lactate is not directly responsible for the deep burn in the muscles. It is the result of a separate reaction.

Below is a picture of the lactic acid chemical compound whose chemical formula is HJC3H5O3. This compound has a hydroxyl group adjacent to the carboxyl group, making it an alpha hydroxy acid (AHA).
The acidosis, or increase in acidity, associated with the increase in lactate concentration during intense exercises is directly correlated with the Glycolysis Energy System. When Adenosine Triphosphate (ATP), the chemical used to fuel the energy pathways, is hydrolysed, a hydrogen ion is released. Hydrolysis is a chemical reaction during which one or more water molecules are split into hydrogen and hydroxide ions which may go on to participate in further reactions.

ATP-derived hydrogen ions are primarily responsible for the decrease in pH. A decrease in acidic level corresponds to an increase in acidity (see pH scale below). During intense exercise, aerobic metabolism cannot produce ATP quickly enough to supply the demands of the muscle. As a result, the Glycolysis Anaerobic Energy System (anaerobic metabolism) becomes the dominant energy producing pathway as it can form ATP at high rates.
Glycolysis, which is the breakdown of carbohydrates, results in the formation of pyruvic acid and hydgrogen ions (H+). A build-up of hydrogen ions increases the acidity of the muscle cells and interferes with their operation. To counter this, carrier molecules called nicotinamide adenine dinucleotide (NAD+) remove the hydrogen ions. The NAD+ is reduced to NADH that deposits 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 H+ and the H+ will build-up in the muscle cell. To prevent the rise in acidity, pyruvic acid accepts H+ forming lactic acid that then dissassociates into lactate and H+. Some of the lactate diffuses into the bloodstream and takes some H+ with it as a way of reducing H+ concentration in the muscle cell.

Due to large amounts of ATP being produced and hydrolysed in a short period of time, the buffering systems of the muscle tissues are overcome, causing pH to fall and creating a state of acidosis. This is a natural process which facilitates the easier disassociation of Oxyhaemoglobin and allows easier transfer of oxygen from the blood.

Lactate Concentration
The normal pH of the muscle cell is 7.1, but if the build up of hydrogen ions continues and the pH is reduced to 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 (deep, burning sensation). This point is known as the lactic threshold (LT), anaerobic threshold (AT) or onset of blood lactate accumulation (OBLA).

The normal amount of lactic acid circulating in the blood is about 1-2 millimoles/litre (mml/l) of blood. The onset of blood lactate accumulation occurs between 2-4 mml/l. In trained athletes, this occurs around 70-80% of VO2 Max. Coaches often build training programs for their athletes based upon their lactate threshold.

The breakdown of glucose or glycogen (carbohydrates - a sugar-based fuel) produces lactate and hydrogen ions. For each lactate molecule, one hydrogen ion is formed. It is the presence of hydrogen ions, not lactate, that makes the muscles acidic and results in the deep burning sensation and the hindrance of muscle function. As hydrogen ion concentrations increase,the blood and muscles become more 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 in the central nervous system. The athlete will experience pain and possible disorientation and nausea.

Sports Coach, Brian Mackenzie
Wikipedia: Lactic Acid, Unknown Author
3Dchem, Karl Harrison

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