Facts and fallacies about muscle soreness...

When it comes to deciding how often you should train, a common rule-of-thumb is never to train a muscle that's still sore from a previous workout. However, a lack of muscle soreness doesn't tell you whether a muscle has recovered completely. In fact, the damage can persist even when the ache has gone away.

A good example comes from research carried in the Journal of the Neurological Sciences [1]. In a group of untrained men, significant soreness was evident for up to three days after exercise. Signs of muscle damage in the blood were higher for up to five days. Muscle function was also impaired for five days.

However, while other symptoms of exercise-induced muscle damage clears within a week, damage to the neuromuscular system (the "chain of command" that transmits signals from the brain to the muscle) lasts for 10 days or more — in untrained men, at least.

Changes to the "efficiency" of your neuromuscular system are one reason why you gain strength very quickly after starting a resistance-training program. In the early 1970's, for example, researchers showed that 100 days of isometric exercise led to a 90% increase in muscle strength. However, there was only a 25% increase in muscle size.

If you do want to avoid feeling sore after exercise, the best way is to ease your way into a new training program gradually. After just one bout of exercise, the repair of muscle damage can take up to two weeks.

In the case of extreme forms of exercise such as a marathon, signs of muscle damage last a lot longer. Once the repair is complete, the muscle can tolerate the same task with less damage and faster recovery.

The bottom line is that muscle soreness is not generally a good indicator of exercise-induced muscle damage [2], and isn't always the best way to decide how often to train a muscle group.

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References
1. Deschenes, M.R., Brewer, R.E., Bush, J.A., McCoy, R.W., Volek, J.S., & Kraemer, W.J. (2000). Neuromuscular disturbance outlasts other symptoms of exercise-induced muscle damage. Journal of the Neurological Sciences, 174, 92-99
2. Nosaka, K., Newton, M., & Sacco, P. (2002). Delayed-onset muscle soreness does not reflect the magnitude of eccentric exercise-induced muscle damage. Scandinavian Journal of Medicine and Science in Sports, 12, 337-346