The relation between sarcomere length and active tension in isolated semitendinosus fibres of the frog

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Abstract
The relation between sarcomere length and tetanic tension at various states of shortening was investigated in single frog semitendinosus fibers subjected to different degrees of prestretch (2.45-3.0 [mu]). The capacity to produce tension changed in a characteristic way during shortening, the tension output at each length being determined by the actual sarcomere spacing without reference to the striation spacing at the onset of contraction. The capacity to shorten against a given load was independent of the initial striation spacing, provided the load was not great enough to fatigue the fiber. The functionally relevant structure of the contractile system of the intact muscle cell may always be in the same state at a given sarcomere length independent of how the previous length change was achieved by passive extension at rest or by active shortening from a prestretched position. This probably means that contraction involves a structural change of the contractile system which is a true reversal of the change produced by passive extensin of the resting fiber. These aspects of the contractile behavior of the intact muscle fiber agree with the concepts of the sliding-filament hypothesis of muscular contraction.