The work that a certain muscle is able to produce depends on how well it is equipped with oxygen-rich blood. If it lacks oxygen, lactic acid is accumulated which, in turn, disturbs the pH balance and microdamages appear. In order to fix the damages, many biochemical processes are started in the organism. One of those processes is the increase of blood to the damaged muscle and the activation of macrophages to remove waste materials out of the organism. Macrophages accumulate around the damaged muscle to help repair the damage.
They increase the concentration of nitrogen oxide, which leads to the widening of blood vessels and the transport of matter that helps to build and regenerate the muscle. If the muscle is better equipped with blood, the transport of oxygen and nutrients will be better, leading to a faster growth of the muscle.
During inaction, the body receives 70% of its energy from fat and 30% from carbohydrates. However, as the intensity of the physical activity increases, the need for ATP (adenosine triphosphate, a molecule rich in energy which is also the basic and only source of energy) goes beyond our body's abilities to deliver the adequate amount of oxygen. In that case, the muscles produce energy via an anaerobic way, i.e. without the use of oxygen and they use glucose as a main substance in order to gain energy. By further decomposing glucose, a final product called pyruvate is made.
If there is not enough oxygen, the anaerobic threshold is crossed where the intensity of the activity is high and the speed at which the pyruvate is made overcomes the speed of oxidative phosphorylation (getting the ATP in an aerobic manner). Once this threshold is crossed, the lactate concentration in blood is significantly increased compared to its amount during inactivity. Pyruvate enters the anaerobic mode of disintegration which product is the lactic acid salt, which can be transferred to lactic acid. This mechanism of gaining energy results in a pH drop in blood and an overall acidity of the organism that is interpreted as pain or fatigue.