One of the common things you hear during a work out is people saying, “Oh, I can feel the lactic acid building”. I always kind of laugh to myself about that because lactic acid is not what is causing the sensation. However, I never want to explain what it is during because (as many of you know) I can be quiet garrulous when it comes to physiology. So grab a pen and paper, I’m going to educate you on the science of what really causes the warm burning sensation that is “lactic acid”.
To explain this as simple as I can I am going to start talking about the glycolytic energy system. When we exercise the main energy source for the body is adenosine triphosphate (ATP) derive from glucose/glycogen aka “blood sugar” (Kenny, Wilmore, & Costill, 2012). For ATP to be created it must go through several biochemical reactions. Theses reactions take place during the process of glycolysis and during the oxidization process of the Kreb’s cycle and Electron Transport Chain. The easiest way to understand these processes is that glycolysis is associated with anaerobic (oxygen deprived) training and Kreb’s (oxygen available) is linked to aerobic training.
Glycolysis is responsible for attaching a Hydrogen ion (H+) to a taxi cab molecule called NAD+ converting it to NADH, which would later be used in the production of ATP by the electron transport chain (Kenny et al., 2012), along with creating small amounts of ATP and Pyruvate molecules.
Traditional, if someone is training aerobically the Pyruvate molecules and NADH will be transported to the mitochondria to become oxidized and converted into ATP in the Kreb’s cycle and the Electron Transport Chain. However, when the body is not able to process these byproducts they start to accumulate within the cell, playing a huge role in the sensation of lactic acid and fatigue.
When the NADH and Pyruvate cannot carry on into the mitochondria they must be broken down. The NADH molecule is oxidized, removing the hydrogen ion that was attached during glycolysis. This converts it back to NAD+ and the hydrogen ion is transferred to the pyruvate to form lactate acid (Mougios, 2007). However, the lactate acid is once again broken down in the bloodstream soon after formation. What is left over from the lactate being broken down are the hydrogen ions. These ions are actually the source of the sensation of fatigue during maximal efforts of exercise. When there is a flux of hydrogen in the muscle cells they inhibit our muscles from contracting consistently. This is also considered to be what is called our lactate threshold (Mougious, 2007).
Consistent aerobic exercise can produce several physiological adaptations to occur to create greater lactate threshold. A major adaptation would be a larger and denser mitochondria. By having larger mitochondria, it leads to several physiological improvements including the ability to process larger amounts of NADH molecules (Ljubicic, 2009). Therefore, the mitochondria will have to ability to convert more NADH molecules into NAD+ in the electron transport chain and Kreb’s Cycle. Opposed to then process of the hydrogen ion being attached to the pyruvate molecules created in glycolysis. Thus, causing an increase in lactate threshold and the ability to work harder for a longer duration of time.
Hopefully, this was informative and not too over the top with scientific terms. If you have any questions about this stuff talk to me after or before any of classes. It is always fun for me to geek out on the exercise physiology.
Thanks for reading!