A velocity‐dependent shortening depression in the development of the force‐velocity relation in frog muscle fibres.

Abstract

SUMMARY1. During the onset of activation in isolated frog muscle fibres the development of the force-velocity (T-V) relation was determined by imposing single and double ramp releases. The experiments were performed at 35-6 0C or 19-22 TC and at a starting sarcomere length of about 2-25 ,um.2. A velocity-and time-dependent shortening deactivation was shown to exist during the development of contraction. It was found that, early during the tetanus rise, at submaximal levels of activation, the values of T (the steady force exerted by the muscle fibres at any velocity of shortening V lower than V0) were significantly affected by previous conditioning shortening. Conditioning shortening at lower speeds led to potentiation of T and, at higher speeds, to depression. Both these effects were independent of the amount of shortening and, in addition, were not present at the tetanus plateau.3. At each given time or isometric tension throughout the tetanus rise the values of T, normalized for those determined at the same velocities at the tetanus plateau, were found to be inversely correlated with the actual velocities of shortening. The slope of this relation (a measure of the velocity-dependent shortening deactivation) decreased exponentially with time, attaining, in six fibres at low temperature, 10 % of its initial value within 26-73 ms.4. The results may be explained in terms of a cross-bridge model of contraction by assuming that the rate of development of activation is controlled by the rate of release of the Ca2+ as well as by the velocity at which the muscle fibres are allowed to shorten and in turn by the actual number of attached cross-bridges.

Proceedings of the Physiological Society, 25‐26 July 1986, Cambridge Meeting: Communications

“…1 shows the results of a typical experiment. They support the hypothesis that during relatively fast lengthening of an active fibre the rates of attachment and detachment of cross-bridges are higher than under isometric conditions (Colomo, Lombardi & Piazzesi, 1986 the finger; this was sometimes produced actively by the subject but more commonly by passive means. There were clear thixotropic changes in stiffness which were dependent on the rest period between stirring and the tap (Fig.…”

In situmuscle power differs without varyingin vitromechanical properties in two insect leg muscles innervated by the same motor neuron

“…In maximally stimulated muscles, force depression most often varies linearly with the magnitude of shortening (Edman, 1975;Marechal and Plaghki, 1979;Herzog and Leonard, 1997;deRuiter et al, 1998;Herzog et al, 1998;Josephson and Stokes, 1999;Meijer, 2002), except during in vivo, voluntary contractions in human knee extensors (independent relationship) (Lee et al, 2000). Although force depression in submaximally stimulated frog and rat muscles does not vary with distance shortened (Colomo et al, 1986;Meijer, 2002), both submaximally stimulated insect muscles 178 and 179 show positive linear relationships between force depression and distance shortened (Fig.·9). Moreover, submaximally stimulated insect and rat muscles exhibit no effect of the initial length on force depression (see Results) (Meijer, 2002), which differs from the positive correlation in tetanically stimulated muscles (Brown and Loeb, 2000;Edman, 1980;Edman et al, 1993;Josephson and Stokes, 1999).…”

Unloaded shortening of skinned mammalian skeletal muscle fibres: effects of the experimental approach and passive force

“…All approaches used for determination of V u are initiated by tetanical stimulation of intact fibres or Ca 2+ activation of skinned fibres under isometric conditions; however, the kind of subsequent isotonic releases differs in various investigations. Different modes of slack tests (Hill, 1970;Claflin et al, 1989), force clamp (isotonic) (Podolosky & Teichholz, 1970;Julian, 1971) or length ramp (isovelocity) experiments (Colomo et al, 1986) are in use for determination of Vu on single muscle fibres. Another factor which could effect Vu is the sarcomere lengthdependent passive force of the fibres at the beginning of the isotonic releases.…”