The stiffness under isotonic releases during a twitch of a frog muscle fibre.

These investigations were undertaken to elucidate whether the difference between the time courses of the isometric tension and the velocity of shortening upon releases to a given isotonic load various times during a twitch is due to a change in the kinetics of the individual cross-bridges, i.e. in the rate constants governing the transitions between the various strongly bound cross-bridge states. Single skeletal muscle fibres from the frog (Rana temporaria, sarcomere length 2.3 microns, temperature 4-6 degrees C) were stimulated to produce isometric twitches and released after various delays to shorten isotonically against finite loads (P). Force, stiffness (K, measured by longitudinal oscillation 0.3-0.8 nm per half-sarcomere peak-to-peak, 2 kHz), and change in length were measured simultaneously. The velocity of shortening (V) was estimated as the quasi steady-state value, i.e. the value attained after the initial oscillatory transient response to the change in length preceding the isotonic shortening. A reduction in P led to a reduction in K, however, the relative reduction in K was not as great as the relative reduction in P. Thus, when extrapolated to P = O, K was about one third of its peak isometric value. Under the assumption that K is to the first order of approximation proportional to the number of strongly bound cross-bridges, P was divided by K to give an estimate of the average load per cross-bridge yo = P/K.V showed an almost linear dependence on yo (correlation coefficients 0.95-0.998). The slope and intercept of this relation (regression analysis) showed no dependence on the delay after stimulation of the release to the isotonic load. The results can be explained if the kinetic properties of the individual strongly bound cross-bridges are unaffected by the level of activation which changes during the course of the contraction.