As strong as a feather: New insights into optimal muscle design

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Are the biggest and most bulging calf muscles necessarily the most powerful? The scientists behind new research into muscle architecture say it's not the size of a muscle that delivers the most force, but rather its layout—in particular, a feathered layout called "pennate."

Associate Professor Christofer Clemente from the University of the Sunshine Coast, Dr. Taylor Dick from the University of Queensland, Dr. Robert Rockenfeller from the University of Koblenz and Dr. Michael Günther from the University of Stuttgart used mathematical models to identify the ultimate muscle design—finding which angle of muscle fibers will best complement a muscle's ratio of length-to-thickness.

Their insights, published today in Royal Society Open Science, could impact the way athletes are trained and robots are built.

"Essentially, the more muscle fibers you have working side by side, the greater the force the muscle can generate," Dr. Clemente said.

"By arranging more fibers at an angle, pennate muscles have an increased cross-sectional area, which is directly related to their ability to produce force.

"While at first glance this appears to be an inefficient design, we know it to be a natural design marvel, perfected by evolution."

The trade-off, however, is that while pennate muscles excel in force production, they aren't as efficient for speed.

While this was already known, this new research identified a second design criterion: the ratio between the length and thickness of muscles.

With the help of mathematical-geometrical models, the researchers were able to identify an optimal pennation angle to match the length-to-thickness ratio of a muscle to enable maximum force production. They called this phenomenon the "pennation mechanical advantage."

Lead author Dr. Rockenfeller said a greater understanding of the pennation angle would inspire new approaches in biomechanics and robotics.

"This knowledge could inform better training regimes for athletes, improve rehabilitation techniques for injuries, and even bioinspire more efficient designs for robots and prosthetics that mimic human movement," he said.

"As we continue to learn more about muscle architecture, it's clear that nature has optimized our muscles in ways that are both sophisticated and incredibly efficient.

"So, the next time you admire a cyclist's powerful calves or a sprinter's explosive start, remember that much of their strength comes down to the clever, pennate design of some of their muscles—a natural engineering feat that's been perfected over millions of years."

More information: Robert Rockenfeller et al, Rethinking the physiological cross-sectional area of skeletal muscle reveals the mechanical advantage of pennation, Royal Society Open Science (2024). DOI: 10.1098/rsos.240037

Journal information: Royal Society Open Science

Provided by University of the Sunshine Coast