Correlation between shortening velocity, force-velocity relation and histochemical fibre-type composition in rat muscles.

Isometric and isotonic contractions of three muscles in the rat hind leg (soleus, extensor digitorum longus (EDL) and peroneus longus (PL] were recorded in situ at 35 degrees C and with nerve stimulation. Additionally, the histochemical muscle fibre-type composition of the three muscles was determined by the method of Guth and Samaha (1970). The data obtained from soleus and EDL muscles were similar to those reported in previous studies. On the basis of twitch contraction time, rate of rise of tetanic tension and maximum shortening velocity, the contraction speed of EDL was 2-3 times higher than in soleus. In the PL muscle, the twitch contraction time, rate of tension rise and shortening velocity were 17 ms, 30 Po/s and 12 muscle fibre lengths/s, respectively; the data showed that the contraction speed of PL muscle was intermediate between that of the soleus and EDL muscles. In the case of soleus, more than 75% of the cross-sectional area was occupied by type 1 (slow) fibres; in both EDL and PL muscles more than 90% of the area was occupied by type 2 (fast fibres). However, the two fast muscles (EDL and PL) had different proportions of type 2B fibres; the area occupied by the type 2B fibre complement was less than 5% in PL, whereas it was around 70% in EDL muscle. The differences in shortening velocity and force-velocity relation among the three muscles could be explained on the basis of their respective muscle fibre-type compositions.