To evaluate individual motor units within a muscle, an electrode must be placed into the muscle itself. Needle electrodes used for this purpose will be described below. As with all work involving electrical equipment, a ground electrode must be used and should be placed on the same extremity that is being investigated. For grounding, a plate electrode is the most useful and best tolerated by the patient, though uninsulated needles have been used.
The electromyographer chooses the type of needle electrode to use, and it is preferable that he or she uses one type consistently to become well acquainted with its characteristics.
Made of stainless steel, the monopolar needle electrode has a very finely sharpened point and is covered with Teflon or other insulating material over its entire length, except for a 0.5 mm exposure at the tip. The needle serves as the active electrode, and a surface electrode placed on the skin close to it serves as a reference.
The main advantage of monopolar needle electrodes is that patients accept them better because they are of small diameter and Teflon covering allows them to slide in and out of the muscle easily. Moving the needle causes less discomfort.
The major disadvantages of this needle is that, with repeated use, the size of the bare tip changes, thereby limiting the number of examinations for which that needle can be used. The Teflon peels back, exposing a larger area that then changes the recorded characteristics of the motor unit potentials. Because the active electrode tip and the surface electrode are separated by some distance, the background noise becomes much greater as remote muscle contractions may be picked up easier.
With the concentric needle consists of a cannula with an insulated wire (or wires) down the middle. The active electrode is the small tip of the center wire, and the reference electrode is the outside cannula. Concentric needles may have two central wires (bipolar), in which case the active and reference electrodes are at the tip and the outside cannula acts as the ground.
Because the active and reference electrodes are closer together, using the concentric electrode minimizes background noise. The electrode picks up motor units from only a very small distance. Another advantage of this electrodes is that no (reference) surface electrode is needed.
The main disadvantage of the concentric electrode is that, by comparison with other needles, its larger diameter can cause more pain, and moving the electrode around is uncomfortable. When one tries to use a small-gauge concentric needle, bending becomes a problem when the needle is dulled by repeated use. This needle can be resharpened with a fine honing stone.
Used for special studies, this needle consists of a 0.5-0.6 mm stainless steel cannula with a 25 µm fine platinum wire inside its hollow shaft. In a side port 3 mm behind its tip, the cut end of the platinum wire is exposed.
Because they can pick up gross motor unit activities, the evaluation of single motor unit potentials with surface electrodes is difficult. They are best used as reference electrodes when monopolar needles are used. They can also be used however, in kinesiology to obtain gross indications about muscle activity and in gait analysis.