A Low-Cost 2-D Sarcomere Model to Demonstrate Titin-Related Mechanisms for Force Production

Given the recently proposed three-filament theory of muscle contraction, we present a low-cost physical sarcomere model aimed at illustrating the role of titin in the production of active force in skeletal muscle. With inexpensive materials, it is possible to illustrate actin-myosin cross-bridge interactions between the thick and thin filaments and demonstrate the two different mechanisms by which titin is thought to contribute to active and passive muscle force. Specifically, the model illustrates how titin, a molecule with springlike properties, may increase its stiffness by binding free calcium upon muscle activation and reducing its extensible length by attaching itself to actin, resulting in the greater force-generating capacity after an active than a passive elongation that has been observed experimentally. The model is simple to build and manipulate, and demonstration to high school students was shown to result in positive perception and improved understanding of the otherwise complex titin-related mechanisms of force production in skeletal and cardiac muscles.