Objective
To assess the abnormal spontaneous activities in needle electromyography (EMG) according to the type of herniated lumbar disc and anteroposterior the diameter of dural sac in magnetic resonance imaging (MRI).

Method
We performed the MRI on 120 patients with low back pain (LBP) and measured the midline anteroposterior diameter of dural sac in a MRI axial view. Fifty patients with a disc protrusion or extrusion in MRI were reviewed for the clinical findings on physical examination and assessed for the abnormal spontaneous activities (ASA) in needle EMG.

Results
Seventy cases with a normal finding in MRI did not have differences in a dural sac diameter regardless their age or sex. Fifty cases with a protrusion or extrusion in MRI showed that the dural sac size decreased more in an extrusion than in a protrusion, and more in a paracentral type than in a central type. The incidence of ASA in a needle EMG increased more in an extrusion than in a protrusion, and more in a paracentral type than in a central type. The incidence also increased according to the decrement of a dural sac diameter. The incidence of ASA were 100% in a group with both motor and sensory signs, 94% in a group with a motor sign, 86% in a group with a sensory sign, 26% in a group without motor or sensory sign.

Conclusion
We concluded that the abnormalities in needle EMG must be correlated with the direction of herniated lumbar disc and anteroposterior diameter of the dural sac in MRI as well as clinical findings.