Electromyographic value of the tibialis posterior muscle.

Differentiation between peroneal palsy and L5 radiculopathy poses a diagnostic challenge. However, clinical weakness of the tibialis posterior muscle indicates that the involvement must lie outside the peroneal territory, potentially at a radicular level. Electromyographically this muscle should provide similar information but it has been relatively ignored, possibly for reasons of presumed inaccessibility. A retrospective study was undertaken to delineate the potential electrical discriminatory value of this muscle. Patients selected were those with clinical evidence of lower extremity neurogenic dysfunction in whom needle electrode examination had been performed on the tibialis posterior as well as other posterior and anterior compartment musculature. The parameters measured were the presence of abnormal spontaneous activity suggestive of denervation plus neurogenic motor unit alterations. Abnormalities were detected in the tibialis posterior which were reflected equally in the anterior but not posterior compartment muscles, indicating a shared L5 radicular innervation. Thus this muscle represents a useful source of electrical information assisting in the differentiation between peroneal and radicular dysfunction.