Vertebrate striated muscle: length dependence of calcium release during contraction.

It is well known that the force-generating ability of vertebrate striated muscle depends upon its length. At lengths longer than the optimum there is a close correlation between decreasing overlap of the myofilaments and the lessening of contractile force, but at lengths shorter than the optimem additional factors may contribute to decreased contracility. For example, the release of calcium into the myofilament space is decreased during contractions at short muscle lengths, and the morphological changes seen in light and electron micrographs of shortening fibers suggest that the decreased calcium release occurs in the core of the fiber. This length dependent change in calcium release may be one of the factors determining the decrease in contractility with shortening.