Here’s how we move, using the elbow as an example. A muscle is attached to the bones on either side of your elbow joint. Inside the muscle, proteins called myosin (red), which are arranged in tiny rows called thick filaments, have little arms that reach out and grab onto proteins called actin (blue), which are arranged in tiny rows called thin filaments. Alternately grabbing, pulling, and releasing, the myosin, like a tug-of-war team, brings the actin on one side closer to the actin on the other side. The shortening of the muscle, which results from this sliding filament mechanism, is called muscle contraction. Because the muscle is attached to each bone by a tendon, the bones are pulled together and the elbow bends.
This arrangement of proteins, like a stack of flattened hexagons, is called a sarcomere. It’s the “functional unit” of muscle contraction, meaning that in theory, if a muscle had just one sarcomere like in the cartoon, it would still work.
That’s the simple version.
The molecules, of course, are shown greatly enlarged. To maximize efficiency, the muscle has an intricate structure of repeating units that will make your head spin. Sarcomeres are attached end-to-end (about 10,000 per inch) to form contractile rods called myofibrils, and myofibrils are stacked side-by-side to fill each muscle cell, which is also known as a muscle fiber. A unique feature of skeletal muscle tissue (the type that can be voluntarily controlled, as in the example) is that the individual cells are extremely long – almost as long as the muscle itself! So a muscle in the arm has “only” around 250,000 muscle fibers – far fewer than the billions of cells one normally finds in an organ. The extreme length of our skeletal muscle cells probably makes them quicker and more efficient.
Now let’s consider the entire muscle again. Its whole purpose is to move a bone through space. To do this, it has to have a stable attachment at one end, called the origin. When the muscle contracts, the other attachment, known as the insertion, is moved closer to the origin, and this is what bends your elbow. Within the muscle, all of the sarcomeres are shortening at once, but the ones closest to the origin are hardly moving at all. Meanwhile the sarcomeres near the insertion are moving rapidly, pulled by the cumulative efforts of all the myosin molecules further up the myofibril, allowing the muscle to win the “tug of war” against gravity.