Force-velocity relation and stiffness in frog single muscle fibres during the rise of tension in an isometric tetanus.

Abstract

The force-velocity (T-V) relation and the force-extension (T1) relation are determined at present times and at increasing isometric tensions during a tetanic contraction in frog single muscle fibres in which the passive compliance in series with the sarcomeres was made very small. The slope of the instantaneous T1 relation, the fibre stiffness, increases roughly in proportion to the level of the rising isometric tension at which the measurements were made. The value of V0 (the velocity of shortening under zero load) is time-independent, whereas the force T exerted during shortening at any velocity V lower than V0 increases gradually with time after the beginning of the tetanus volley and attains its steady state level before the isometric tension has attained the tetanus plateau and the fibre stiffness its final value. It is concluded that the delay of the development of the isometric tension and of the fibre stiffness with respect to the development of the T-V relation is determined by a specific factor of the contractile process. It is interesting to note that in a cross-bridge model of contraction, in which the value of the rate constant for cross-bridge formation is moderate, the recruitment of actin sites which is measured by the characteristics of the instantaneous T-V relation, is expected to lead significantly the actual cross-bridge formation, which is measured both by the instantaneous isometric tension and by the instantaneous stiffness.