Temperature sensitivity of tension development in a fast-twitch muscle of the rat

Isometric contractions of the biceps brachii (short head) muscle in the rat were recorded in vitro with direct stimulation and at different temperatures (range, 35 degrees C-10 degrees C). In confirmation of our previous findings from fast extensor digitorum longus and slow soleus muscles, the time and rate parameters of the twitch and the tetanus showed an increased temperature sensitivity below 20 degrees C. The dependence on the initial muscle length of the rate of rise of tetanic tension was examined at 27 degrees C and at 15 degrees C. When represented as a percentage of the tetanic tension at each length, the rate of rise was independent of muscle length at both temperatures. Our interpretation of this particular observation is that the increased cooling depression of the rate of tension rise below 20 degrees C is not associated with a qualitative change in its underlying basis.