Spatial relationships in electrostimulation: application to electromagnetic field standards.

Spatial relationships in electrostimulation are examined with a myelinated nerve model. Excitation thresholds are determined for: 1) terminated axon within a locally constant electric field; 2) bent axon within a locally constant field; and 3) a field of finite extent over the affected axon. For application to electromagnetic field standards, the minimum excitation threshold of the in situ electric field applies to a straight, terminated axon; a field measurement averaging distance of 5 mm is recommended.