1. Rest and equilibrium length muscle sarcomeres are composed of thin filaments (actin) which traverse the sarcomeres from the Z membranes up to the H band; at this level the filaments are considerably thicker and less numerous.
2. Shortening of muscle is associated with a transformation of thin into thick filaments in the A band.
3. These observations are discussed in terms of interaction of actin and myosin to form a supercoiled structure as the basis of contraction.