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The 'protective' message in sore muscles

When we try out a new exercise or push ourselves hard, we know that we will tire our muscles, creating  tiny tears in the fibres, and we are going to feel sore later on. 

One advantage to enduring such discomfort is the knowledge that it can signal muscle growth, but a new study, published in Nature Communications, reveals that there's more to the message of muscle soreness than we realise.

The research has explored how our muscles recover from fatigue and may provide insight into the difference between healthy and unhealthy muscles.

"We found the muscle itself has a protective mechanism stopping an individual from further damaging themselves in the days after exercise," said Dr Bradley Launikonis from University of Queensland's School of Biomedical Science and an Australian Research Council fellow.

Launikonis and his team took small biopsies from the thighs of participants to test human muscle fibres at three points in an exercise cycle, which involved eccentric or stretch contractions (where the muscle is being contracted and stretched at the same time, like running downhill, squatting, doing pushups or pullups).

"We mapped the muscle structure before an individual exercised, as well as 24 and 48 hours after," Launikonis explained.

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What they found was intriguing. 

They knew that eccentric contractions cause a "significant increase in the muscle calcium content". This increase leaves the muscles prone to damage - the increase in calcium can potentially activate proteins called calpains which play a role in muscle wasting conditions

"This type of activity (stretch contractions) is also what is causes permanent damage in muscular dystrophy," Launikoni said. "Permanent scarring is left in the muscles, replacing the normal muscle tissues, and overall making the boys with MD weaker (progressively weaker as these events continue to happen through their childhood years, and into the teens)."

But this was not the case for healthy muscles.

"We found how healthy muscle protect themselves from calcium-induced damage in the two days after heavy exercise, while the muscles are sore," Launikoni explained. 

We become sore while our bodies warn us off pushing too hard, and then once the muscle is less vulnerable to damage we feel better and ready to exercise again.

"The soreness a person feels is the body saying it is fatigued, that the muscles are vulnerable, and it's time to rest," he says.

The mechanism at play is the result of small cavities called vacuoles inside the muscle fibres where the increased calcium - that would otherwise cause damage - accumulates. Once the muscle repairs, the vacuoles disappear, Launikonis explained, until our next bout of hard exercise.

Unless you try to push through the pain, which Launikonis believes would be a bad idea.

"I think that the vacuoles that form and accumulate calcium reduce the ability of the muscle to produce its otherwise high level of force," he said. "Calcium is a necessary regulator of the force response, so its reduced levels inside the muscle fibres caps the ability to mechanically damage the muscle with tension. 

"But if someone managed to ignore the pain and keep going, I'd imagine they would put their muscles at risk of significant damage."

The temporary discomfort then appears to be a process designed to help us.

"It tells us human muscles are very adaptive and can protect themselves," he says.

But it also means researchers may be able to use the insights to explore new treatments for degenerative conditions. 

"It is possible we can learn from healthy muscles, and start looking at muscular dystrophy now, to see if the protective mechanisms acting in healthy muscles are in action in muscular dystrophy," Launikonis says. "If not, we could have an exciting new research direction (a starting point)."