By use of an optical system, with which the beam of the first-order diffraction line of He-Ne laser light from a single frog skeletal muscle fiber was split by the wedge-shaped mirror to be focused on two photodiodes ( Haugen & Sten - Knudsen , 1976), small changes in sarcomere length (less than 1 A) could be recorded during quick fiber length changes (up to 1.2% of Lo, complete within 0.2-0.4 msec) applied at the plateau of isometric tetanus. Data were only obtained on fibers which showed typical sinusoidal sarcomere length changes in response to sinusoidal fiber length changes during tetanus with a linear relation between their magnitudes. Measurements of sarcomere length changes were made at various points along the fiber length. The interval between the onset of fiber length changes at one fiber end and that of force change recorded at the fixed fiber end was explained by the propagation of mechanical impulse at about 180 m/sec. In the case of quick releases, the onset of sarcomere shortening near the fixed fiber end tended to take place after that of force change, especially with long fibers, indicating that the drop in force during a quick release may not always be associated with sarcomere shortening along the entire fiber length. This implies that the force changes in response to rapid length changes may not give correct information about the cross-bridge properties. Irrespective of the point at which sarcomere shortening was recorded, it was always observed that the onset of quick force recovery occurred while sarcomere shortening was still in progress. Such a phenomenon can be simulated by a viscoelastic multi-segment model with series elasticity located in each segment.