That all too familiar burning sensation you feel when training is the bind of many an athlete! The causes are important so that you can accordingly to delay the onset. Lactic acid is always the scape goat here, and to say it’s unfunded is an understatement. Lactic acid is actually there to help; in fact its part of its very own energy system aptly called ‘the lactic acid system’. The lactic acid system is designed to liberate more ATP (needed for energy production) in a process known as anaerobic glycolysis, this occurs when oxygen has either ran out or wasn’t present in the first place.
Glycolysis (breakdown of glucose for energy) produces energy as well as pyruvate and Hydrogen ions, and it’s the Hydrogen ions that increase the acidity of your blood and thus causes the ‘burn’ you feel as you fatigue!
McArdle, W, D., Katch, F, I, & Katch, V, L. (2007). Exercise Physiology: Energy, Nutrition and Human Performance. 6th Ed. Lippincock Williams & Wilkins.