Muscle stiffness changes during enhancement and deficit of isometric force in response to slow length changes.

The mechanism of the enhancement and the deficit of isometric force in response to slow length changes in tetanized frog muscle fibers was studied by recording the stiffness changes with sinusoidal vibrations (0.5-1.9 kHz, peak-to-peak amplitude 0.1% of L0). When a tetanized fiber was slowly stretched, the fiber stiffness first rose abruptly and then decreased linearly while the stretch went on; after the completion of stretch, the stiffness decreased towards a steady value which was equal to that during the ordinary isometric tetanus at the same fiber length, though the force decayed towards a steady level higher than that of the ordinary isometric tetanus at the same fiber length. This indicates that the enhancement of isometric force after stretch is associated with decreased stiffness. If, on the other hand, a tetanized fiber was slowly released, the force and the stiffness changed in parallel with each other. Recordings of the segmental length changes along the fiber with a high-speed video system (200 frames/s) indicated that all the segments lengthened in response to slow stretch, while the segmental length changes in response to slow release was markedly nonuniform. These results are discussed in connection with the cross-bridge performance and the filament-lattice structures.