Muscles ache
Muscle need energy to work. They get it by using oxygen to break down a chemical fuel called glucose, a form of sugar, which is delivered to them by the blood. If a muscle is working hard, it starts to run short of oxygen and it cannot break down glucose in the normal way. Instead, glucose is turned into lactic acid. The acid builds up inside the muscle and makes it ache. If the muscle is allowed to rest, the acid is broken down and the ache disappears.