A refined technique of electromyography and electronic analysis was utilized to determine the responses of several muscle groups to passive stretch. Distinctive responses were obtained in the soleus and long head of the triceps humeri. In the normal person these had a definite relationship with the phase of the stretch cycle, were about fifty microvolts in amplitude initially, and diminished on repeated stretching. In patients with disease characterized by hyperirritability of the skeletal musculature, the same responses were obtained in the same muscles. However, in these subjects the initial amplitude averaged 100 microvolts and repeated stretching produced a marked increase in amplitude and duration of the discharge. In all subjects the responses were confined to multiple discrete areas within the muscle bulk. The gastrocnemius and the lateral head of the triceps were silent to repeated stretching. A few scattered non-potentiating discharges were found in the anterior tibial muscle of some of the diseased patients.

The possible sources of error in the investigation are considered and discussed in relation to the findings. These results correlated well with tentative criteria for the presence of tonic muscle fibers, based on recent indentification of tonic anterior horn cells in the cat. Further investigation in clinical syndromes seems warranted. The activity in the patients with neuromuscular disorders appeared to be analogous to the activity of the tonic motoneurons in decerebrate preparations. The experimental results indicate some duality of function in the skeletal musculature of the human being.