Maximal shortening velocities, isomyosins and fibre types in soleus muscle of mice, rats and guinea-pigs.

1. Guinea-pig soleus contains only type I fibres and slow isomyosin, SM2. Rat and mouse soleus contain about 70% of type I fibres and a mixture of isomyosins: slow, SM2 and intermediate, IM. Many rat soleus muscles contain a third isomyosin of a slow type, SM1. 2. The maximal velocity of unloaded shortening, V0, is largest in mouse soleus (6.11 Lf s-1), slowest in guinea-pig soleus (1.67 Lf s-1) and intermediate in rat soleus (4.16 Lf s-1) (Lf = fibre length). 3. In guinea-pig soleus, V0 is equal to the maximal velocity (Vmax) computed using the Hill force-velocity relationship; V0 is approximately twice as large as Vmax in mouse and rat soleus. 4. V0 measures the unloaded shortening velocity of the fastest fibres whereas Vmax is a function of the force-velocity characteristics of all the fibres contained in the muscle. 5. V0 increases according to the isomyosin composition of the fibres in the sequence SM2 less than SM1 + IM less than IM.