Fig. 4. Sample recordings from the slack test measurements of V₀, the unloaded velocity of shortening from a red muscle preparation. (A) Three superimposed recordings of tetanic force before and after the length steps (L) shown in B. The fibre bundle was released by different distances from the same starting length. (C) Recordings of force from A shown on expanded scales and with regression lines fitted to the early part of the force redevelopment. The broken vertical lines marks the time of the release. The horizontal arrow marks t, the time from the release to the regression intercept for the largest step. This is the time required to take up the slack following the length step. (D) Relationship between step size, L, and t. The slope of the regression line, 1.503±0.002 L₀ s^-1 (mean ± S.E.M., N=3), is V₀ for this fibre bundle. The intercept on the L axis, 0.053±0.0001L₀ (mean ± S.E.M. N=3), is an estimate of the compliance of the series elastic component (SEC) of this fibre bundle. L₀, the fibre length at which maximum isometric force was produced, was 8.7 mm.