Shortening velocity extrapolated to zero load and unloaded shortening velocity of whole rat skeletal muscle.

The shortening velocity at zero load (Vmax) extrapolated from velocities measured during isotonic releases was compared with unloaded shortening velocity (V0) determined by the slack test. Experiments were performed in vitro at 20 degrees C on soleus muscles from rats. The Vmax was 3.2 +/- 0.1 (mean +/- S.E. of mean, n = 10) fibre lengths/s while the V0 determined for the same muscles was 5.0 +/- 0.1 fibre lengths/s. The ratio of V0/Vmax was 1.6 +/- 0.1. Soleus muscles of the rat are heterogeneous with respect to the intrinsic shortening velocities of their fibres. The results suggest that V0 is a measure of the unloaded shortening velocity of the fastest fibres whereas Vmax is a function of the force-velocity characteristics of all the fibres within a skeletal muscle preparation.