A long thin protein that wraps around actin filaments in sarcomeres (the basic contractile unit of muscle). When the muscle is inactive, it is this way (except for rare situations like rigor mortis) because tropomyosin is blocking the sites on the actin where Myosin needs to bind to generate the force that moves our muscles. When the muscle is activated, a protein called troponin causes a change in shape (allosteric change) that reveals the binding sites.