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Background: Needle EMG may be negative in mild or predominantly sensory lumbosacral radiculopathies. In such cases, an increase in the latency of the soleus H-reflex is a useful diagnostic criterion for establishing sensory fiber compromise at the S1 root level. However, if clinical signs of radicular involvement are lacking, the latency of the H-reflex is normal. We therefore studied the recruitment curve of the soleus H-reflex to investigate whether a change in the electrical threshold for eliciting the H-reflex might be a more sensitive criterion for detecting subclinical S1 root dysfunction.Methods: Clinical and electrophysiological findings from 26 patients with chronic back pain and radiculopathy were compared with data obtained from 40 healthy subjects.Results: An increase in the mean H-reflex threshold was the only abnormal electrophysiological finding in patients with no clinical sign of root injury (58%). A decrease in the mean H-reflex amplitude and a prolongation of H-reflex latency was observed in patients with radicular signs (42%). In both patients groups, F-wave and needle EMG studies were normal. No radiological evidence of S1 root compression was found.Conclusions: The study of the recruitment curve of the soleus H-reflex may be usefully associated to F-wave and needle EMG studies to detect possible S1 root dysfunction in mild lumbosacral radiculopathies. An increase in H-threshold may be the earliest abnormality in the absence of focal neurological signs. [ABSTRACT FROM AUTHOR]