There is a lot of theories about why we get sore after exercise. And many of them are debunked by modern research. Diving into this subject can be confusing since there are so many factors included in keeping the body in homeostasis (balance). We will consider some of them below.
Lactic acid (lactate)
Lactic theory has been turned on its head. Previously we thought that lactic acid was the major culprit in training. When we experience exhaustion, we call it “the lactic threshold” because the concentration of lactic acid increases significantly in a short time. However, lactate, being lactic acid without acid, has actually been shown to provide new energy to the muscles during long distance training. It passes by the liver and is converted into glucose which is used as energy in the muscles. This is called the Cori cycle.
Lactic acid is associated with exhaustion since it increases significantly when we are exhausted, but is in actuality something positive and something we really want to utilize fully. It is not a cause of fatigue, but a protection from fatigue.
And also, something that might surprise many of you, even though lactic acid give acidosis, it is not a cause of acidosis, but a protection from it. This is explained below.
Even the acid theory has been threatened. There is no doubt that excess acid in the muscles is problematic for the nerves and can cause pain, but when muscles are exhausted, it is not the excess acid in general which is exhausting the muscles. It is excess acid from specific processes that causes problems. When we reach exhaustion there is a surplus of K+ ions in the extracellular fluid, outside the muscle cells. K+ reduces muscle contraction and during exhaustion, there is so much accumulated K+ the muscles are unable to contract. However, the lactic acid and CO2 will also have an acidifying effect, but still their function is to protect against exhaustion. This has been shown in other studies that also mention that increased CO2 in the blood gives a higher lactate threshold.
The white circle shows what happens to muscle strength when lactic acid is injected into a muscle that has been exhausted. Muscle strength returns quickly.
The chemical process that is producing lactic acid is protecting us agains acidosis by using two H+ protons (the acid) to produce an end product with one H+.
Also, scientists have see that the metabolic acidosis usually is fully recovered already within 20 minutes after ending the exercise. The acidosis might be a reason for the burning sensation in the muscles during exercise, but it is not a cause for increased muscle soreness 2-4 days after the exercise.
Inflammation theory has also been scrutinized. It was earlier believed that the soreness comes from micro tears in the muscles that activate immune and inflammatory mechanisms. But this does not seem to be true either.
There are many inflammation factors in the body but there are not many that correlate to pain progression during muscle soreness. The main inflammatory marker in studies of the immune system response is CRP, and it does not change after training. Various other inflammation factors increase and decrease at different times after training, so one can not really say that inflammation is a direct cause of the pain.
But there is no doubt about one thing when it comes to muscle soreness: it hurts and it hurts the most during day 2 and 3.
The best theory of pain related to DOMS is that it is a central sensitization, ie an increase in the sensitivity in the brain and the central nervous system. Therefore, even small movements or the slightest pressure on the sore muscles is painful. The causes of central sensitization are often complex, and may include acidity, inflammation, micro injuries to muscles, oxidative stress, etc.
This is also the reason why we experience less pain and become less stiff from the workout if we continue with it. The nervous system adapts to (read: becomes confident in) the challenge and thereby we do not experience so much pain. But if we do a new workout or increase the amount of exercise, we get DOMS again, because the nervous system needs time to accept this new challenge.
There are large individual differences in how intensely central sensitization appears. It can be anything from genetic differences to childhood challenges. When talking about pain, it is always very individual and relative. Sometimes you can get DOMS after exercise simply because you have a stressful period at work, problems in your relationships, or other things that make people more susceptible to pain conditions and sensitization of the nervous system.
But no matter what is your ground zero, it is possible to prevent aches and pains if you take care of some basic needs: proper nutrition, a proper amount of pure water, good sleep and a proper amount of exercise (avoid overtraining).
And proper breathing. Using recovery breathing daily will result in an obvious boost in your recovery.