We rely on our muscles for all kinds of movement. Muscles function as biological motors within the body because they generate force and produce movement. Some of these movements are obvious—running, climbing stairs, raising your hand, playing the piano. We are less aware of others—moving food through the digestive tract, breathing in and out, pumping blood through the body. A muscle’s ability to produce movement depends on electrically excitable muscle cells containing proteins that can be activated by the nervous system.

The contractile machinery of muscles is ancient. The proteins underlying muscle contraction are found in eukaryotes that lived more than 1 billion years ago, and the first muscle fibers common to all animals are found in cnidarians (jellyfish and sea anemones), which evolved at least 600 million years ago. Thus, basic features of muscle organization and function are conserved across the vast diversity of eukaryotes. As we will discuss, the geometry and organization of these proteins largely determines how muscles contract to produce force and movement.