Why Work Makes Muscles Grow

Flexing bicep muscles on a man.
A flexing bicep. (Image credit: Aaron Amat, Shutterstock)

As any bodybuilder can attest, muscles grow when we make them do more work. Now, new research explains how muscle cells translate weight-lifting overload into bulk.

The secret lies with a chemical factor produced by muscle cells during work (such as during weight lifting) that signals muscle stem cells to multiply and take on the load. The substance, serum response factor (Srf), apparently triggers muscle stem cells — dormant cells capable of differentiating into muscle cells — to proliferate and become muscle fibers. More muscle fibers means bigger overall muscles and more strength.

The findings may lead to new ways to combat muscle atrophy associated with age and illness, according to study researcher Athanassia Sotiropoulos, of the medical research institute Inserm in France.

"This signal from the muscle fiber controls stem cell behavior and participation in muscle growth," Sotiropoulos said in a statement. "It is unexpected and quite interesting."

Using mice that were genetically engineered to lack Srf in their muscles, the researchers found that without the factor, overloading the muscles does nothing to boost growth.

Srf sends its signals via a network of genes, including one called Cox2. Anti-inflammatory medicines such as ibuprofen blunt Cox2, raising the possibility that these drugs might inhibit muscle growth, Sotiropoulos said.

Most likely, she said, therapies aimed at boosting muscle growth (for example, after a long period of bed rest) would be aimed at this complex web of chemicals working under Srf.

The research appears today (Jan. 3) in the journal Cell Metabolism.

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Stephanie Pappas
Live Science Contributor

Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.