A model of the sarcomere is described, which is based upon (a) the sliding filament theory ofHuxley, (b) the ultrastructure of the sarcomere, (c) the biochemical mechanisms responsible for mechanical coupling and uncoupling of myosin and actin, (d) the constancy of the volume of the sarcomere and (e) a statistical analysis of random forces acting on myosin cross-bridges, constrained by flexible and extensible heavy meromyosin subfragments. The results can be fitted to experimental Frank-Starling diagrams. Maximal velocity of contraction, derived from the theory, is independent of sarcomere length, while the rate of force generation is predicted to increase with increasing sarcomere length. The relationship of the new model with the older three-element models is discussed and a new definition of the series elastic element is proposed representing the series component as being distributed over the thin and elongated heavy meromyosin subfractions which connect the crossbridges to the myosin backbone.