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Department of Physiology, University of Michigan, Ann Arbor 48109.

1. In intact skeletal muscle fibres, estimates of unloaded shortening velocity obtained from slack test measurements (V0) have been shown to exceed, by approximately 7%, estimates obtained from extrapolation of velocities measured during isotonic releases (Vmax). In contrast, published values for the V0 of whole soleus muscles of rats exceed Vmax by 56%. In the present study, we tested the hypothesis that this difference between whole muscles and single fibres is due to a difference in their respective force-velocity relationships at loads less than 5% of maximum isometric tetanic force (P0). In addition, we examined, by computer simulation, the effect of inter-fibre heterogeneity on the force-velocity characteristics of a whole muscle. 2. The force-velocity relationship of soleus muscles of rats was determined at low loads, in vitro at 20 degrees C, by recording force maintained during controlled shortening at constant velocities. The relationship was simulated by assigning a hyperbolic force-velocity curve to each motor unit and summing the force contributions of individual units at each of a series of velocities. 3. When measurements from low loads were included, the force-velocity relationship intersected the velocity axis at V0 (5.0 +/- 0.1 fibre lengths/s, mean +/- S.E.M., n = 10), not Vmax (3.1 +/- 0.1 fibre lengths/s). The simulated and measured force-velocity relationships agreed at all loads, supporting the premise that the deviation from hyperbolic form responsible for the large disparity between V0 and Vmax of whole muscles is a consequence of heterogeneity in shortening velocity among fibres.

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