Sarcomere length and tension changes in tetanized frog muscle fibers after quick stretches and releases.

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Abstract
The sarcomere length changes in tetanized frog muscle fibers in response to quick fiber length changes were examined along the fiber length with a high-sensitivity laser diffraction technique. The experiments were only performed with muscle fibers in which the uniform orientation and sarcomere length of the component myofibrils were well preserved during a tetanus. When the sarcomere length changes were recorded near the fixed fiber end, the delay of the onset of sarcomere length change in response to the applied fiber length change tended to be longer than that of the onset of tension changes recorded at the fixed fiber end. The magnitude of sarcomere length changes was larger near the moving fiber end than near the fixed fiber end. In the case of quick releases, the resulting sarcomere shortening tended to outlast the fiber shortening, so that the quick tension recovery started during the sarcomere shortening. These results indicate (i) that the tension changes in response to quick fiber length changes may not give direct information about the cross-bridge properties and (ii) that the viscoelastic multisegmental nature of muscle fibers should be taken into consideration in interpreting the tension responses to quick length changes.