To address this question, let’s look at a muscle in microscopic view. As you can see in figure 13, muscles are bundles of long, thin cells, called fibers. Each fiber, in turn, is made up of thousands of strands, fibrils, that in turn contain thousands of banded structures called sarcomeres (Greek for “flesh component”). Sarcomeres generate pulling forces because they are made of two key proteins—one thin, the other thick—that try to slide past each other like interlacing the fingers of your two hands. This contractile action occurs whenever a nerve sends an electrical signal to the muscle,
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