Force-velocity relation and isomyosins in soleus muscles from two strains of mice (C57 and NMRI).

We compared soleus muscles from two strains of mice, NMRI and C57. Soleus muscles from NMRI mice produced slower twitches and lower maximum tetanic force (Fo) but higher maximum tetanic stress (So), (owing to their smaller weight). Their Hill's velocity constant (b) was lower, but their force constant (a/So), their maximum velocity of unloaded shortening (Vu) and their maximal mechanical power (Pmax) were similar. All soleus muscles contained two isomyosins (SM2 and IM) and the two myosin heavy chains (MHC1 and MHC2A) corresponding to type I fibres and type IIA fibres; however, soleus muscles from NMRI strain had higher proportions of isomyosin SM2 and of myosin heavy chain 2A. Regression equations were computed between the mechanical variables and the myosin heavy chain content. Using a simple hypothesis, the results were used to estimate the mechanical properties of type I and type IIA fibres. We conclude that type IIA fibres from soleus muscle are mechanically more similar to slow-twitch type I fibres than to fast-twitch type II fibres. The results also suggest a hypothesis to account for the diversity of isomyosins, by a matching diversity of mechanical properties based on a separate physiological control of the three factors that control Pmax.