Histochemical characteristics and contractile properties of the spinotrapezius muscle in the rat and the mouse.

In the spinotrapezius muscle of the rat and the mouse approximately equal proportions of A,B and C fibres are present. The spinotrapezius therefore contains a lower proportion of B fibres than are known to be present in soleus, but a higher proportion than in tibialis anterior. These results are consistent with the functional properties of the muscles, for the values for isometric twitch contraction time, the half relaxation time, and the twitch summation frequency of the spinotrapezius are also intermediate between those of soleus and tibialis anterior. The present experiments support the view that the proportion of B fibres to A and C fibres govern the overall functional characteristics of mammalian muscles.