Contraction kinetics of striated muscle fibres following quick changes in load

Abstract

The contraction kinetics of single striated muscle fibers and small fiber bundles from the frog and the toad were measured when the load was changed from Po to L < Pn. Simultaneous recordings were made of displacement at one end and force at the other end of the preparation. After the load was changed, the contractile force generally reached a steady value before the contraction velocity became steady. The amount of time required for isotonic contraction to become steady depended on the change in fractional load and on the temperature; it did not depend on sarcomere length in the range 2.2-3.0 [mu] or on the number of fibers in the preparation. The characteristics of non-steady state are described in terms of the displacement deviation (the difference between the actual displacement at a given time and the back extrapolation of the steady phase of the displacement rocord) and the null times (the times at which the displacement deviation became zero, measured relative to the time at which the contractil force first reached the value of the load). The time average of the transient velocity was approximately equal to the final steady velocity. The product of the null time following a given relative force step and Vmax, the steady velocity of unloaded contraction, was found to be independent of temperature. This is taken as e vide re e that the isotonic velocity transients originate In the contractile mechanism. The non-steady state following step changes in load is identified with the motion of cyclic contraction mechanisms. The motion of the specific model formulated by A. F. Huxley (1957) was compared with that of frog muscle fibers and, although the transients in the two systems differ in detail, the characteristic dimensions are of the same order.