These experiments examine length's and activation's influence on contraction of the sarcomere in living heart muscle and single heart muscle cells. Length's influence on the sarcomere shortening dynamics and the time course of truly isometric contraction will be explored in an isolated intact heart muscle preparation. Changes in sarcomere length and the spacings of internal markers delineating segments of tissue will be measured and directly compared to investigate the influence of heterogeneity at short lengths. The dynamics of isometric contraction will then be observed when a region's sarcomere length is controlled directly while observing the spacing of the tissue markers, and vice versa, to determine the nature of coupling between sarcomeres and surrounding tissue. The dynamics of properties of shortening muscle will be measured at specific sarcomere lengths when force is controlled at steady and non steady levels. The experiments will be repeated in muscle tetanized in various levels of external calcium to evaluate activation and time's influence on the respective shapes of the sarcomere length-force and sarcomere length-velocity relations. Individual sarcomeres will be inspected microscopically during contraction in intact single heart muscle cells and the effect of length, activation, and time upon peak shortening velocity reexamined. The possibility that a signigicant internal load exists within the partially activated sarcomere will be tested directly. Techniques will be developed to extend the microscopic observations in living cells to the ultrastructure and myofilament disposition within the partially activated sarcomere.