Dr. Robert Fritz, Medical Director of the Center for Health, Sports Medicine, Nutrition, Sports Science, and Training Therapy, Sportordination Vienna: It's possible, but we don't know the answer. Science is still unsure. Over the past few years, the theory on muscle soreness has changed significantly. There have been many theories about the cause of muscle soreness, but most of them have now been disproved. There is now an exciting new approach, but currently very little scientific evidence to support this theory. However, it is not entirely far-fetched, and it may well be that neural microdamage is a cause, or at least a contributing factor, of muscle soreness.

The first attempts to explain muscle soreness argued that it was caused by hyperacidity of the muscle due to lactate. Studies showed that people with high lactate levels were more likely to experience muscle soreness. However, it then became apparent that there are also people without high lactate levels who still experience muscle soreness. Conversely, there were also people with high lactate levels who did not experience muscle soreness. This disproved the link between lactate levels and muscle soreness.

Then there were theories about fluid balance with electrolyte shifts. It is now generally agreed that muscle soreness is caused by excessive mechanical stress that overwhelms the muscle's capacity to withstand stress. This leads to a damage mechanism.

This raises the big question of where this damage comes from. The latest theory pursued in this regard states that microtrauma occurs, i.e., small tears in the ultrastructure of the muscles, i.e., in the Z-discs. Muscle soreness is more likely to occur during eccentric exercise, i.e., when the muscle is stretched particularly strongly. However, it is no longer certain whether this is true. We know that muscle soreness occurs particularly during eccentric exercise, such as walking down stairs or hiking downhill, i.e., movements that demand more from the muscle than it can actively control. When climbing stairs, you only go as far as the muscle actively climbs. However, when going down stairs and the muscle is tired, it still has to cushion the body's weight and becomes overloaded, otherwise you will fall. This leads to excessive strain. And that could lead to injury in the ultrastructure of the muscle.

If you look at muscle soreness in an MRI scan, you can see intramuscular edema, i.e., fluid accumulation in the muscle. This is swelling. This could happen when the muscle tears, as well as for many other reasons. Since muscle soreness only lasts for about two to three days, edema could well be the cause of the pain.

The development of edema, in turn, could have two causes. The first theory is based on the assumption that an injury, fluid influx, and inflammation occur. Taken together, this leads to pressure in the muscle, which triggers the pain.

The second, completely new theory states that it is a compression axonopathy. The aforementioned pressure in the muscle compresses the nerve endings to such an extent that the nerve could be damaged.

The scientific term for muscle soreness is “delayed onset muscle soreness” (DOMS). Muscle soreness actually occurs with a delay and is not acute like pain from other injuries, such as when you twist your ankle. The theory is that this is related to fluid accumulation in the muscle, or edema, which cannot develop as quickly. Another theory comes from evolutionary biology and states that the pain is delayed so that you do not feel pain in an emergency situation and can still escape. From an evolutionary perspective, this could have been essential for survival when hunting for food, for example.

What we know for sure is that muscle soreness actually has few consequences, as it does not cause any lasting damage. This is now widely accepted, even though it used to be taught that it leads to scarring of the muscle. Since even microdamage to the muscles should actually leave tiny scars, the theory of neural microdamage could well be given some weight: this would not cause scarring. However, the researchers who put forward this theory themselves emphasize that their statement is still purely hypothetical and that proof is still pending.

For sports medicine, it is important to know whether muscle soreness is dangerous. It is certainly not dangerous, even if it is terribly unpleasant. Muscle soreness only indicates that you have overworked the muscle, but there is no need to fear it. That is why it is important to start small when training and to increase the intensity slowly.

Recorded by Stefanie Flunkert