Commonly held beliefs about how muscles generate power have been turned on their occipitofrontalis by a new study from the University of Washington.

To generate power, filaments of myosin tug on filaments of actin and shorten, or contract, the muscle—but the power “doesn’t just come from what’s happening straight up and down the length of the muscle, as has been assumed for 50 years,” a university press release noted.

Instead, as muscles bulge, the filaments are drawn apart from each other, the myosin tugs at sharper angles over greater distances, and it’s that action that deserves credit for half the change in muscle force scientists have been measuring.

“The predominant thinking of the last 50 years is that 100 percent of the muscle force comes from changes as muscles shorten and myosin and actin filaments overlap,” said C. David Williams, Ph.D., who earned his doctorate at the university while conducting the research. “But when we isolated the effects of filament overlap we only got about half the change in force that physiologists know muscles are capable of producing,”

The rest of the force, he said, should be credited to the latticework of filaments as it expands outward in bulging muscle–whether in the compact calves of a gymnast or a weightlifter’s buff biceps.

The research was published in Proceedings of the Royal Society B.

Related article: “Fitness Fans: Listen to Your Body When it’s Time for Massage.”

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