Isometric endurance in fast and slow muscles in the cat

By use of a method of sequential stimulation described previously, the ability to sustain tensions of 3--100% of the initial strength (maximum isometric tension that could be developed in the fresh muscle) to fatigue (endurance) was assessed in the soleus (slow-twitch), medial gastrocnemius (mixed, fast-twitch), and plantaris (mainly fast-twitch) muscles. For all fractions of the initial strength the endurance was longest in soleus and shortest in plantaris. However, although plantaris fatigued at any tension examined, soleus could maintain a tension of up to 30% of its initial strength indefinitely with no sign of fatigue. Part of the difference in endurance between these two muscles could be accounted for in terms of blood flow because arterial occlusion sharply reduced the endurance of soleus but had only a small effect on endurance in plantaris. However, even with an occluded circulation, soleus still had substantially longer endurance than either medial gastrocnemius or plantaris. The origin of fatigue in any of the three muscles examined appeared to be in the muscle; there was no evidence of failure of transmission at the neuromuscular junction.