We all know that feeling- from running too far, too fast. Or from pedaling a bike up a hill really hard. (If you don’t, you might want to get out more….) That nauseating feeling in your muscles that makes you stop and lingers for a short while is due to the build-up of lactic acid, which is simply a byproduct of the muscular metabolic process.
Muscle contraction begins with a signal from your brain, which propagates through your nervous system and causes the release of the neurotransmitter acetylcholine at the neuromuscular junction. When the acetylcholine binds to its receptors a muscle, a series of intracellular events occurs and the muscle fibers contract. The energy source for this contraction is called ATP. During normal exercise, glucose (sugar) and oxygen produce a sufficient amount of ATP to fuel muscle contractions (via the Krebs Cycle). However, during extreme efforts like sprinting, our bodies require more energy and a different energy cycle is revved up to produce much greater volumes of ATP for a short amount of time. Each of these cycles of energy production produces pyruvic acid, a product of glucose breakdown. At moderate levels of exercise, moderate levels of pyruvic acid are produced and much of it goes into the Krebs Cycle to produce energy; at high levels of exercise, high levels of pyruvic acid are produced.
Pyruvic acid is converted into lactic acid and at high levels of exercise, more pyruvic acid is produced than the body can handle and a buildup of lactic acid results. At a certain point, our body can clear lactic acid at the same rate that it is being made. This is called the lactate threshold and it is a very important aspect of training for elite athletes. The higher their lactate threshold, the longer they can go at a high tempo before the build-up of lactic acid brings on burning and muscle fatigue.
A process called the lactate shuttle is important because it allows for some of the lactic acid to be oxidized and converted into additional energy for the muscles. Efficient clearance of lactate is likely an important factor in lactate threshold and it has been suggested that lactate may ultimately contribute to the prevention of fatigue.
The burning sensation associated with the build-up of lactic acid is due to the presence of hydrogen ions in the blood. The hydrogen ions released by the lactic acid may act locally to indicate fatigue, but may also act on peripheral nervous system receptors to send the signal to the brain that glucose is being utilized at a high rate and muscle fatigue is setting in. In this way, the central nervous system (which initiated the muscle contraction in the first place) regulates the physical activity.