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Biceps bulge, calves curve, 50-year-old assumptions muscled aside

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Posted July 10, 2013
C. David Williams created a 3-D computer model of filaments of myosin (in red) reaching out and tugging along filaments of actin (in blue, looking like stands of pearls twined together) during the contraction of a muscle. The model allowed researchers to consider the geometry and physics at work on the filaments when a muscle bulges. Credit: D Williams/U of Washington

C. David Williams created a 3-D computer model of filaments of myosin (in red) reaching out and tugging along filaments of actin (in blue, looking like stands of pearls twined together) during the contraction of a muscle. The model allowed researchers to consider the geometry and physics at work on the filaments when a muscle bulges. Credit: D Williams/U of Washington

The basics of how a muscle generates power remain the same: Filaments of myosin tugging on filaments of actin 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.

Instead, University of Washington-led research shows that 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. 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,” said C. David Williams, who earned his doctorate at the UW while conducting the research.

The rest of the force, he said, should be credited to the lattice work of filaments as it expands outward in bulging muscle – whether in a body builder’s buff biceps or the calves of a sinewy marathon runner.

Williams, now a postdoctoral researcher at Harvard University, is lead author of a paper appearing online July 10 in the Royal Society journal Proceedings of the Royal Society B.

“One of the major discoveries that David Williams brought to light is that force is generated in multiple directions, not just along the long axis of muscle as everyone thinks, but also in the radial direction. This aspect of muscle force generation has flown under the radar for decades and is now becoming a critical feature of our understanding of normal and pathological aspects of muscle,” said Thomas Daniel, UW professor of biology, one of Williams’ advisers and co-author on the paper.

Read more at: Phys.org

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