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

1. The relation between sarcomere length and tetanic tension at various states of shortening was investigated in single frog semitendinosus fibres that were subjected to different degrees of prestretch (2.45-3.0 mu).2. 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.3. The capacity to shorten against a given load was independent of the initial striation spacing, provided the load was not great enough to cause fatigue of the fibre.4. The findings strongly suggest that the functionally relevant structure of the contractile system of the intact muscle cell is always in the same state at a given sarcomere length independent of how the previous length change has been 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, at least in so far as it is of relevance to function, is a true reversal of the change produced by passive extension of the resting fibre. These aspects of the contractile behaviour of the intact muscle fibre are in full accord with the concepts of the sliding-filament hypothesis of muscular contraction.