You know that burning feeling you get with muscle fatigue? It’s lactic acid build-up, right? Wrong, it’s actually the hydrogen ion in the lactic acid. When your body makes lactic acid, it splits into lactate ion (lactate) and hydrogen ion. Hydrogen ion is the acid in lactic acid. It interferes with electrical signals in your muscles and nerves, slows energy reactions, and impairs muscle contractions. So, the burn you feel in intense exercise is caused by hydrogen ion build-up. Any muscle cramps you may get while exercising aren’t down to lactic acid either; most muscle cramps are caused by muscle nervous receptors that become ‘over-excitable’ with muscle fatigue.
Whenever you use carbohydrates, a significant portion is converted to lactate; this lactate is then used in the same tissues as fuel, or it is transported to other tissues via the blood stream and used for energy. During intense strength training you are using carbs for fuel at a rapid rate, this accelerates lactic acid production, which builds up in your muscles and blood because it can't be used as fuel fast enough. However, if you slow down the pace of exercise or stop exercising, the rate of lactate used for energy soon catches up with the rate of lactate production.
During endurance races, such as marathons and triathlons, blood lactic acid levels stabilize even though lactic acid production increases. This is due to the production of lactic acid being matched by the body’s ability to use it as fuel. As the intensity of this type of exercise increases, the heart, slow-twitch muscle fibers, and breathing muscles prefer lactate as a fuel.
Why is lactic acid so important in metabolic regulation? The exact answer is unknown, but there do appear to be several physiological reasons. Lactic acid, in contrast to glucose and other fuels, is smaller and better exchanged between tissues. It moves across cell membranes by a rapid process called facilitated transport. Other fuels need slower carrier systems such as insulin.
Athletes need both high intensity and over-distance training to improve their capacity to use lactic acid as a fuel during exercise and recovery. High intensity training develops the cardiovascular capacity required for efficient lactic acid transport to tissues, so it can be used as fuel. Over distance training causes tissue enzyme adaptations that increase the use of fatty acids for energy; this helps slow lactic acid production from carbohydrates and enhances the tissues ability to use the lactic acid as fuel.