A study has been carried out of the tension responses of single fibres from frog semitendinosus muscle at 4 degrees C which occur in response to rapid lengthening and shortening steps throughout the isometric tetanus. 2. Instantaneous stiffness values were calculated as the ratio of the tension change to the length change. 3. Stiffness was seen to increase more rapidly than the tension during the initial development of force and was constant during the tetanus plateau. During the relaxation phase of tension, stiffness declined more slowly than the force up to the shoulder; thereafter both declined rapidly and with a nearly similar time course. 4. Throughout the initial rise of force, the period of maximum force development and the early phase of relaxation, a rapid stretch produced a significantly greater tension change than a rapid release. The fibres appeared 'stiffer' during a stretch. 5. The experimental results are shown to be consistent with the suggestion that cross-bridges may detach during a small rapid shortening step but remain attached during a similar lengthening step.
