Development of force-velocity relation, stiffness and isometric tension in frog single muscle fibres

The force-velocity (T-V) relation and the force-extension (T1) relation from single fibres isolated from the muscle tibialis anterior of the frog were determined at present times during the rise of tension and the plateau of an isometric tetanus. During the rise of an isometric tetanus the value of V0 (the velocity of shortening at zero load) remained constant, whereas both the force T exerted during shortening at a given velocity lower than V0 and the slope of the instantaneous T1 curve increased with time after the beginning of the stimulus volley. However, while the value of T (a measure of the level of activation) attained its final value much before the isometric tension attained the plateau, the slope of the T1 curve (the fibre stiffness, a measure of the number of attached crossbridges) increased throughout the course of tension development. In six muscle fibres, at 2.25 microns sarcomere length and at about 4 degrees C, at a time during the tetanus rise when T had attained 81% of its final value, the fibre stiffness and the isometric tension had risen, respectively, to 50% and to 38% of the values attained at the tetanus plateau. Later, when T had already attained 99% of its final value, the fibre stiffness and the isometric tension had risen, respectively, to 80% and to 76% of their plateau values. In the same muscle fibres, the average value for the amount of step release required to drop the plateau tetanic tension to zero was only 8.62 nm (observed) or 6.16 nm (extrapolated from the linear part of the T1 curves) per half sarcomere. It is concluded that the large delay in the development of the isometric tension with respect to the development of T, and therefore with respect to the development of the whole T-V relation, cannot be attributed to the passive series compliance of the muscle fibres. It is likely that this delay is due to a specific event in the contractile process.