Question: Why do muscles burn when doing sport?

LauraAnne Furlong answered on 13 Nov 2016:

When we exercise our body uses several different sources of energy, and this depends on how long you are exercising for and how intense it is. It was often said that a waste product of one of the sources of energy, called lactic acid, is responsible for the burning sensation. Athletes would then use tolerance training so they could still perform even when it was there.

However, we now know that the reason for this burning occurs before the lactic acid accumulates in your body. A molecule called adenosine triphosphate stores energy in our body. Every time one of these molecules splits to release energy, a hydrogen ion (a proton) is also released, and the accumulation of these ions lowers your blood pH and this makes it acidic. The nerves in your muscles are sensitive to this, and that is where the burning feeling comes from. Pyruvate molecules bond with the protons to form lactate, so the lactic acid is actually a result of the acidic response/burning in the muscle, rather than the cause.

Some of the other scientists can give a more detailed breakdown of how this all happens, but for the type of experiments I do and area of sport science I work in, this is as much as I need to know 🙂