Delayed onset muscle soreness (sensory nerve)

Delayed onset muscle soreness (DOMS) is pain or discomfort associated with a previously-exercised muscle. It is usually absent immediately after exercise and only appears the day after exercising. It tends to peak at approximately 48 hours post-workout. In addition to pain or discomfort at rest, the muscle is often more sensitive to touch and to movement. Strangely, despite decades of research, we still don’t really know what causes it. One recent hypothesis that has been gathering support is that DOMS is in fact caused by minor, reversible, sensory nerve damage.

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#1. Sensory nerve hypothesis

Recently, a group of researchers suggested that DOMS was not caused by changes inside the muscle fiber, but in fact reflected minor, reversible, sensory nerve damage caused by the compression of muscle spindle nerve endings inside the muscle during eccentric contractions, since these involve very high intramuscular pressures.

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#2. Sensory nerve hypothesis testing

When this hypothesis was devised, there was some indirect evidence to support it. For example, some research had shown that motor nerve conduction velocity reduces after eccentric training workouts. Reductions in nerve conduction velocity typically occur after minor, reversible, nerve damage. Such findings could be taken as indirect evidence by way of comparison (if motor nerve conduction velocity reduces, then sensory nerve conduction velocity might also reduce) and also as indirect evidence by way of downstream effects (motor nerve conduction velocity might be reduced due to reduced input from Ia afferent nerves through the stretch reflex).

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Similarly, some research has also shown that motor nerve diameter reduces after eccentric training workouts. Again, this could be taken as indirect evidence by way of comparison (if motor nerve diameter reduces, then sensory nerve diameter might also reduce). Like nerve conduction velocity, nerve diameter tends to reduce when nerves experience minor, reversible damage.

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One additional observation that adds weight to the idea that minor, reversible nerve damage occurs after very damaging eccentric training workouts is that atrophy has sometimes been observed. While such atrophy could be explained by muscle fibers turning necrotic and not subsequently regenerating, it might also be explained by the presence of minor, reversible nerve damage, because muscle atrophy often occurs in such situations. Moreover, this explanation becomes more likely when little sign of structural muscle fiber damage is observed in tandem with the muscular atrophy that occurs (as has been occasionally reported).

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Even so, the most recent research suggests that while both sensory and motor nerve conduction velocities do reduce after eccentric training workouts, such reductions are similar after initial bouts (which cause severe DOMS) and repeated bouts (which cause only moderate DOMS). This implies that minor, reversible, sensory nerve damage is probably not the primary mechanism causing DOMS. 

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Conclusions

Delayed onset muscle soreness (DOMS) is pain and/or discomfort associated with a previously-exercised muscle. It is usually largely absent immediately after exercise and only appears the day after exercising. It frequently peaks approximately 48 hours post-workout. Oddly, despite decades of research, there is no widely-accepted underlying mechanism for explaining what causes DOMS. One recent hypothesis is that DOMS is produced by minor, reversible, sensory nerve damage. In support of this hypothesis are the observations that motor nerve conduction velocity, motor nerve fiber diameter, and myelin sheath thickness reduce after eccentric training workouts. Similarly, the loss of muscle mass after an eccentric training workout (without severe structural muscle damage simultaneously occurring) implies the presence of minor, reversible, nerve damage. However, recent research has shown that while both sensory and motor nerve conduction velocities do reduce after eccentric training workouts, such reductions are similar after initial bouts (which cause severe DOMS) and repeated bouts (which cause only moderate DOMS). This implies that minor, reversible, sensory nerve damage is probably not the primary mechanism causing DOMS.​