Short-range stiffness of slow fibers and twitch fibers in reptilian muscle.

The semitendinous muscle of the lizard Tilique contains both slow and twitch fibers; by subdivision of its motor nerve, fibers of each type may be stimulated separately. When, during repetitive stimulation of nerve filaments, the muscle was lengthened or shortened, the tension changes included an initial short-range stiffness, followed by a later compliance. With increasing velocities of movement, the short-range stiffness increased toward a limiting value. For slow fibers this limiting value was reached with lower velocities of movement than for the twitch fibers. Provided that the same velocity of movement was used and the movements began from similar initial isometric tensions, the slow fibers resisted the movements with a greater stiffness than the twitch fibers. It is suggested that not all of the observed differences between the two fiber types can be interpreted simply in terms of differences in rates of formation and breakdown of cross-links.