The maximum velocity of shortening during the early phases of the contraction in frog single muscle fibres.

The maximum velocity of shortening (Vmax) was determined at preset times during the development and the plateau of isometric tetani in single fibres isolated from the tibialis anterior muscle of the frog. Experiments were performed at low temperature (3.6-6 degrees C) and at about 2.25 micron sarcomere length. The controlled velocity release method was used. Vmax was measured by determining the lowest velocity of release required to keep the tension at zero. Extreme care was taken in dissection and mounting of the fibres in order to make the passive series compliance very small. The value of Vmax at the end of the latent period for the development of isometric tension (at 4.5 degrees C about 10 ms after the beginning of the stimulus volley) was already the same as later during either the tension rise or at the plateau of isometric tetani. These results show that the value of Vmax of intact fibres is independent of time and activation subsequent to the latent period, and suggest that the cycling rate of the crossbridges may thus attain its steady-state value just at the end of the isometric latent period.