What characteristic of subdermal needle electrodes results in higher impedance compared to surface electrodes?

The characteristic of subdermal needle electrodes that leads to higher impedance, compared to surface electrodes, is their smaller surface area. Subdermal needle electrodes penetrate the skin and provide a more focused point of contact with the tissue, which results in a reduced surface area compared to the larger flat surface of surface electrodes.

When evaluating electrode performance, impedance is influenced significantly by the contact area with the conductive medium (skin and underlying tissues). The smaller surface area of subdermal electrodes makes them less effective at reducing the impedance caused by the layers of skin that present resistance to electrical flow. Hence, when using subdermal needle electrodes, one can expect higher impedance levels due to this limitation in surface area compared to the more extensive contact provided by surface electrodes.