The Maximum Velocity of Shortening of Whole Skeletal Muscles (Soleus, Contractile Properties, Rat, Mechanics).

Abstract

The maximum shortening velocity of a skeletal muscle preparation is traditionally estimated by extrapolating to the velocity at zero load (V(,max)) along a hyperbola fitted to velocities measured at non-zero loads. Unloaded shortening velocity (V(,o)) can be determined by applying a series of step releases of different magnitudes to a fully activated muscle preparation and measuring the times required to remove the imposed slack. V(,o) and V(,max) are equivalent in single skeletal muscle fibers but have not been compared in whole muscles which are heterogeneous with respect to the intrinsic shortening velocities of their fibers. The purpose of this study was to assess the accuracy of estimates made by extrapolating the force-velocity relationship of heterogeneous skeletal muscles along a hyperbola to very low loads, including zero load. Three hypotheses were tested: (1) force produced during release at a constant velocity (isovelocity release) equivalent to V(,max) is greater than zero, (2) V(,o) is greater than V(,max), and (3) during isovelocity release, the minimum velocity required to produce zero force (V(,o)') is not different from V(,o). V(,max), V(,o), and V(,o)' were determined in vitro at 20(DEGREES)C for each of 10 soleus muscles isolated from 4 week old female rats. V(,max) was extrapolated from a hyperbola fitted to 13 velocities measured during isotonic releases at loads ranging from 5 to 50% of maximum isometric force (P(,o)). V(,o) was determined by applying 18 step releases ranging in amplitude from 7 to 12% of optimum fiber length. V(,o)' was determined by measuring forces produced during isovelocity releases at velocities ranging from 40 to 190% of V(,max). Velocity results were: V(,max) = 3.0 (+OR-) 0.1; V(,o) = 5.0 (+OR-) 0.1; and V(,o)' = 4.9 (+OR-) 0.1 fiber lengths/s (means (+OR-) SEM, n = 10). Force produced during isovelocity release at V = V(,max) was 1.5 (+OR-) 0.1% of P(,o). The three hypotheses tested were accepted. At very low loads, force-velocity measurements from heterogeneous skeletal muscles did not fall on the hyperbola fitted to velocities measured at higher loads. The relationship instead followed a more sharply curved path which intersected the velocity axis at V(,o), not V(,max). V(,max) had no apparent physical manifestation. A computer simulation confirmed that this non-hyperbolic behavior could be attributed to heterogeneity among fibers rather than an intrinsic property of the fibers.