While a great deal is known about the structural and physiological properties of muscle as thick and thin filaments slide in the direction to reduce their overlap, very little is known about the structural changes when they slide in the opposite direction and become doubly overlapped. This is ironic because double overlap is the only physiological range relevant to contraction in individual cardiac muscle cells, and much of the physiological range of skeletal muscle also includes the region in which thick filaments not only overlap thin filaments, but thin filaments overlap one another. We test the latter possibility by examining the time-resolved changes in separation of thick and thin filaments as well as the ratio of their associated mass. We have made measurements of the [1,0] and [1,1] equatorial reflections when the filaments slide because of passive compression, and compared these with sliding when the motive force originates in cyclical interactions of the myosin cross-bridges in active muscle. The potential significance of our experiments extends to the question that leads to the discovery of filament sliding in the 1950's. Why does a second set of dense lines appear and then disappear in a muscle sarcomere as it shortens and then re-extends?
