A comparison of muscle stiffness measurements obtained with rapid releases or stretches of frog semitendinosus fibers.

The stiffness-tension relationship has been studied during the isometric twitch. All experiments were carried out at 0 degrees C using small fiber bundles from frog semitendinosus. Rapid shortening and lengthening steps of 3-6 nm/hs, complete in 500 mu sec were given at various times during the twitch. Instantaneous stiffness was measured as the ratio of delta P to delta L and expressed as a relative change to the maximum values recorded at the plateau of an isometric tetanus. The time course of the stiffness changes followed the tension for both releases and stretches. However, the stiffness measured with a rapid stretch was higher than when measured with a rapid release. This raises the possibility that the instantaneous elasticity is non-linear in stretches.