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

In rabbit, rat and human skinned skeletal muscle fibres the length-time relationship of isotonic releases was determined after maximal Ca2+ activation. Slack test experiments provided information about unloaded conditions. Force clamp experiments of different load were extrapolated for zero load and compared with the slack test data. The course length-time relationship for unloaded conditions was similar using both approaches. However, slack test data showed a triphasic shape which could be fitted by three straight lines (phase I, II, III), whereas the data of force clamp experiments exhibited a steady curved shape. Consequently, the instantaneous slopes differed in the two relationships, but the distance which was shortened during the time interval of phase II was similar in both approaches. The ratio between these unloaded shortening velocities resulting from force clamp and slack test experiments was 1.01 +/- 0.05 (SD) (n = 25). The effects of passive force on the velocity of fibre shortening was investigated in skinned rabbit muscle fibres using slack test experiments. A significant increase in the unloaded shortening velocity was observed when the sarcomere length of the fibres was increased to values which exhibited considerable amounts of passive force. The high reproducibility of the isotonic releases required in this study was achieved by improving some methodological details. Using these improved techniques an identity between the relative fibre and sarcomere shortening was observed during the isotonic releases.