Delayed onset muscle soreness (fascia)

Despite a great deal of research being performed into the nature of the delayed onset muscle soreness (DOMS) that occurs in the days after strength training workouts, we still do not know exactly what causes it.

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#1. Why is DOMS not caused by muscle damage?

We are fairly certain that DOMS is not caused by muscle damage for several reasons. Firstly, there is a weak relationship between post-workout fatigue and DOMS. For example, although workouts involving higher volumes typically always cause more fatigue, they do not always cause more DOMS.

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Secondly, the repeated bout effect works differently for DOMS than for fatigue. DOMS experiences a repeated bout effect from the first workout to the second workout, but does not experience any further repeated bout effects from the second workout onwards. In contrast, fatigue seems to experience repeated bout effects (that are smaller in size) across all subsequent workouts.

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Thirdly, the time course of fatigue, muscle swelling, and muscle damage are very different from the time course of DOMS. DOMS follows a characteristic time course, rising to a peak at between 24-72 hours post-workout, depending on the characteristics of the study. In contrast, fatigue usually always peaks between 0 and 24 hours post-workout.

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Moreover, the durations of the DOMS, post-workout fatigue, and swelling can vary greatly compared to one another. Often, post-workout fatigue outlasts the other two phenomena quite substantially.

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The exact time course of muscle damage is difficult to describe because various events take place and each of them have a different timeline.

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#2. How might DOMS be caused by fascial inflammation?

Early studies showed that (muscle) collagen breakdown occurs after eccentric training workouts. Markers of collagen breakdown were detected in urine and in blood samples that were taken during the post-workout recovery period.

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Later research clarified that the collagen surrounding the muscle fiber (the endomysium) is likely broken down and remodeled as a result of eccentric training workouts. Indeed, there is an early increase in the levels of agents of collagen breakdown (such as matrix metalloproteinases) followed by a subsequent increase in the quantity of type III and IV collagen types in the endomysium. 

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Recently, researchers have shown that the fascia surrounding muscles is sensitized to pain caused by electrical stimulation in the post-workout recovery period, in much the same way as muscle tissue. Nevertheless, the increases in fascial sensitivity are more closely related to DOMS than the increases in muscle sensitivity. This suggests that it is changes in the fascia that are instrumental in producing DOMS after the workout, and not changes in the muscle.

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Additionally, recent research has shown that the fascia surrounding muscles increases in thickness in the post-workout recovery period, and that this change in extracellular connective tissue thickness is related to DOMS for part of the time. This also suggests that it is changes in the fascia that cause DOMS after the workout.

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Conclusions

Despite a lot of research being done into delayed onset muscle soreness (DOMS), we still do not know exactly what causes it. Indeed, research shows that DOMS is likely not caused by muscle damage for at least three reasons: [1] higher volume workouts that cause more fatigue do not necessarily cause more DOMS, [2] the repeated bout effect occurs only after the first workout for DOMS but occurs both after the first workout and also after successive workouts for post-workout fatigue, and [3] the time course of DOMS after a workout is very different from the time courses of post-workout fatigue and the development of myofibrillar damage (since DOMS peaks at approximately 48 hours after the workout, while fatigue peaks immediately post-workout, and muscle damage develops progressively over several days afterwards and involves multiple stages). For this reason, researchers have considered whether DOMS could be caused by damage to the extracellular connective tissue (fascia) of the muscle and its subsequent inflammation. Indeed, research has shown that the connective tissue is damaged and remodeled after eccentric contractions, that it increases in thickness during the post-workout recovery period (and that this is related to DOMS), and that increases in the electrical pain threshold of the fascia in the post-workout recovery period are related to DOMS.