What causes DOMS?
A common misconception for the cause of DOMS is that it is linked to lactic acid accumulation and toxic metabolic waste build-up. However, this is now considered to be an outdated theory, as stated by the American College of Sports Medicine. Indeed, the jury is still out as to what the exact mechanism(s) of DOMS are, with a 2003 paper in Sports Medicine proposing that there are up to six hypothesized theories. However, there is the widely held belief that DOMS “appears to be a product of inflammation caused by microscopic tears in the connective tissue elements that sensitize nociceptors and thereby heighten the sensations of pain,” as is stated in a 2013 study from the Strength & Conditioning Journal. Simply put, DOMS appears to occur due to connective tissue microtrauma. The main culprit for said trauma is believed to be due to eccentric muscle contraction—the lengthening or stretching phase—which is know to damage both the individual muscle fibers and connective tissues that surround the muscle fibers. After this microscopic muscle injury, immune cells go to the site of injury to repair the damage—which results in the inflammation and pain on experiences, but also means that muscle fibres will become stronger through the healing process, resulting in a stronger muscle in general. This process can result from any eccentric exercise—not just weightlifting. That includes things like running and yoga, as well as exercises that you simply aren’t used to.
So, does the repair work done by the immune cells and subsequent fortification of the muscle fibres mean that DOMS is a sign that one is becoming stronger and progressing? The short answer is ‘no’, though it is a little more complex than that. Simply put, muscle damage is a contributing factor to muscle hypertrophy (an increase in muscle size through an increase in the size of its component cells), but it’s not absolutely essential. Hypertrophy can occur from mechanical tension and/or metabolic stress. Therefore, though DOMS can provide a general indication that some degree of muscle tissue damage has taken place, it is not a definitive measure for it. What this means is that you don’t have to experience muscle soreness after a workout in order to build muscle, and it isn’t a reliable indicator of productiveness.