A structural and kinetic model of actomyosin interaction in a contracting muscle fibre has been proposed, which is based on the assumption that the myosin molecular motor generates the force in two steps. Initially the nonstereospecifically attached myosin head rolls at the actin surface and stereospecifically locks on actin. Then the alpha-helical lever arm of the myosin molecule tilts around its catalytic domain. The model also includes the modern scheme of ATP hydrolysis by actomyosin. The results of modeling are presented, which quantitatively reproduce all experimentally observed characteristics of responses of tension and stiffness of muscle fibres to T-jumps of different amplitudes.