The smallest functional unit of a myofibril of striated muscle, the sarcomere, is shown here with Z lines or disks at either end. Scaled to tens of thousands of times its actual size, this moving model graphically illustrates the sliding filament theory of skeletal muscle contraction. Reflecting the most recent advances in electron microscopic ultrastructure, the model unequivocally demonstrates that individual myofilaments neither change shape nor shorten — rather the proteins actin and myosin slide past one another, resulting in the shortening of the sarcomere itself. The arrangement, configuration, and function of thin filaments (actin, troponin, tropomyosin), and thick filaments (myosin II), the M-line and Z-line are all clearly depicted. Applications for this model include physiology classes, athletic trainer education, sports medicine specialists, physical and massage therapists, and more. The working sarcomere model helps bridge the link between cellular physiology and muscle tone, muscle cramps, knots and tears. Model also features a translucent sheath representing the sarcoplasmic reticulum, which can be wrapped around the sarcomere to offer greater understanding of membrane action potentials and the role calcium ion transport plays in muscle contraction. Includes a user manual.