1. The hypothesis that the pattern of activity of muscle can determine its contractile properties was tested in the case of the rat soleus subjected to chronic disuse. 2. In order to characterize the pattern of motor activity over long periods of time, a method of chronic EMG recording with indwelling electrodes was developed. Some characteristics of the EMG recorded from unrestrained animals are described. 3. Immobilization of the knee and ankle joints reduced aggregate EMG activity in the soleus to 5‐15% of control and resulted in a shift from tonic to a more phasic pattern of firing. 4. After 4 weeks of immobilization, speeding of the soleus mechanical properties was indicated by: shortened contraction time; decreased tetanus/twitch ratio; increased maximum rate of development of tetanic tension; and decreased fusion during a 5/sec tetanus. 5. Because the contractile properties of the soleus can be altered by a period of disuse with no change in innervation, neuronal control of the contractile mechanism depends in part on impulse activity. © 1969 The Physiological Society
