Why Do Muscles Become Sore After Exercise?

Why do muscles become sore after exercise? An interview with Dr. David Aguayo, a muscle physiologist who works in the Kieser Research Department.


The word muscle comes from the Latin "musculus" and means little mouse. Perhaps the person who gave them the name "muscles" thought they looked like mice running around under the skin when flexed. Try contracting your biceps and you may understand the analogy. So why do muscles become sore after exercise?

Sore muscles – the result of lactic acid?

“Definitely not,” says Dr. David Aguayo, a muscle physiologist who works in the Kieser Research Department. “The pain is caused not by a build-up of lactic acid but is evidence of a micro inflammation. If you do a physical workout that is more strenuous than usual, this can cause microscopic damage to muscle fibres and the muscles becomes sore within 24-48 hours of the workout. In contrast, lactic acid merely reduces pH levels in the cell. This can restrict the ability of muscle fibres to generate strength but does not cause sore muscles.”

Sore muscles – are they essential for muscle build-up?

“From a physiological perspective, there is no direct linkage between muscle build-up and sore muscles,” stresses Dr. Aguayo. “Rather, it is evidence that you have put undue strain on a muscle. It is important, therefore, to give the muscle sufficient time to regenerate.” As a rule, the muscle soreness disappears within four days. However, this does not mean that muscle regeneration is complete. The regeneration period can take several weeks. As a result, our expert recommends that if muscles are extremely sore, you should avoiding working those muscles at the same high intensity. "There needs to be a balance between exertion and regeneration,” says Aguayo. “If you interrupt the repair process and train at an intensity that triggers further muscle damage, you can change the balance in favour of degeneration.” If that happens, muscle tissue disappears and is replaced by fat and connective tissue. It is far better to put a light load on those muscles or to train different muscles and muscle groups.

Does stretching prevent sore muscles?

Does muscle stretching before or during training help prevent sore muscles? “We now know that this is not the case,” says Dr. Aguayo. “Admittedly, stretching can help reduce the perception of pain if muscles are already sore.”

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