The depressive effect of shortening on the mechanical properties of the chick anterior latissimus dorsi muscle was assessed by the isovelocity method during tetanic stimulation where the muscle was subjected to a standard conditioning shortening (7% optimum length at approximately 2% maximum shortening velocity, V0) immediately prior to the isovelocity test. Comparison was made to the mechanical properties of non-conditioned (control) contractions. For any given test isovelocity shortening rate, the observed force was always lower if it was preceded by a conditioning shortening. The percentage difference in isovelocity force between conditioned and control contractions was independent of the test shortening velocity. This suggests that shortening lowered the peak isometric force (F0), but did not affect the shape of the force-velocity relation if normalized to a lower F0. 2. Velocity of unloaded shortening, independently measured by the 'slack' method, was unaffected by the conditioning shortening. 3. The magnitude of the force deficit was diminished if the velocity of the conditioning shortening was increased. 4. Recovery of the force deficit was evaluated by allowing a variable period of isometric force redevelopment between the end of the conditioning shortening and the onset of the test isovelocity shortening. The isovelocity force of the conditioned contraction was less than the corresponding control at all times. Complete recovery was not observed with up to 5 s of additional stimulation. However, recovery was observed if the muscle was allowed to relax briefly. 5. Several possible interpretations of our results were considered. Our results are consistent with the hypothesis that the effect of shortening results from non-uniform sarcomere shortening due to a pre-existing heterogeneity of sarcomere strengths. 6. An Appendix describes: (1) how the force-velocity relation would be affected due to the presence of sarcomere strength heterogeneity, and (2) how a model muscle consisting of a heterogeneous population of sarcomeres would be expected to behave following different types of shortenings.
