This happens due to the formation of lactic acid in his muscles. The body uses oxygen for breaking down glucose in order to obtain energy. Now, during intense exercise, there may not be enough oxygen available to complete this process, so a substance called lactate or lactic acid is made. But this substance can build up in the muscles and the acidity it provokes starts affecting the enzimes that break down glucose yo obtain energy. This gives a sore sensation and fatigue.
