Modeling Muscles From the Inside Out

A new model of skeletal muscle starts from the micro-mechanical properties of the smallest possible unit—the sarcomere—and builds up to the muscle fibers and then to the muscles themselves. In addition, it places the fibers in their natural context—within surrounding soft tissue. The effort brings a new degree of flexibility and realism to muscle simulation.  
“The idea behind micromechanical modeling is to imitate the behavior of the material as well as possible,” says lead researcher Markus Böl, PhD, professor of mechanics of polymers and biomaterials at the Braunschweig University of Technology in Germany. “We’re trying to include all the micro-parameters we can. In this way we do not have to fit the material behavior to the experimental data.” His work appears in the October 2008 issue of Computer Methods in Biomechanics and Biomedical Engineering.
Scientists started making mathematical models of muscles in the 1920s. Most attempts to date were one-dimensional, and they ignored the soft tissue surrounding muscle fibers, Böl says. Also, they usually were built from the outside in: Scientists would look at the way a muscle behaved and tweak their model’s parameters (such as the number of contractions per second) until it matched the behavior. This led to some accurate but limited simulations.
Böl’s work builds muscles from the inside out. He uses the finite element method, originally developed by aerospace engineers to design planes, to divide a muscle into discrete parts that each behave differently. Previous finite element muscle models used a continuum-based approach, which lumped all muscle fibers together and treated them as a single unit. But Böl gets into the nitty-gritty of each tiny fiber. In essence, his modeled muscles behave like a bunch of ropes of different thicknesses attached at the same point. Because the model describes each rope independently, Böl can plug in any parameters he wants and get realistic behavior back out.
In his model, Böl splits the muscle into an active element (the contractile muscle fibers) and a passive one (the incompressible tissue that surrounds them). Putting the “ropes” into the realistic environment of soft tissue yields a more complete picture, he says.
The model has both experimental and clinical value, Böl says. Scientists will use it to test the properties of living muscle, or to help doctors design unique treatments for patients, he believes. He is now working with sports doctors to refine and implement his approach. “But I have to say, these are first trials and work is still in progress,” he cautions.
The new model can simulate any biological tissue that contracts, not just skeletal muscles, says Ellen Kuhl, PhD, professor of mechanical engineering at Stanford University. Kuhl was so impressed that she is now working with Böl to model heart tissue, with the goal of helping researchers develop a patch to replace dead tissue after a heart attack. “I think the cardiac application is even more sexy, because many more people could benefit from it,” she says.

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