Indentations in the terminal cisternae were measured in freeze-fracture replicas of extensor digitorum longus (EDL) and soleus muscles from normal rats and from rats in which the spinal cord had been transected at the midthoracic level 6 weeks earlier. The density of indentations was compared with contraction time and characteristics of K-contractures. In paraplegic rats, the average soleus contraction time decreased from 89 to 46 msec and the average number of indentations, per micrometer of terminal cisternae, increased from 0.9 to 5.0. The EDL contraction time decreased from 35 to 31 msec and the indentations increased from 7.3 to 8.1. In spite of the considerable scatter in the data, there was a clear correlation between indentation density and contraction time. Similarly, indentation density was correlated with the voltage dependence of tension generated during K-contractures. The results show that the density of indentations can be modulated by afferent neural activity and support the hypothesis that indentations play an important role in excitation-contraction coupling.