Page 315 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

292 BIOMECHANICS OF THE HUMAN BODY

skin to low impedance output of the amplifier, and reduce motion artifact (Hagemann et al., 1985).
Dry electrodes are often used during motion analysis for the suppression of motion artifact. Gelled
electrodes use an electrolytic gel or paste as a chemical interface between the skin and electrodes.
The electrode-electrolyte interface can reduce the electrode-skin impedance and improve signal quality
under static conditions, but it also leads to significant increases in signal artifact when mechanically
perturbed (e.g., limb movement, impact during heel strike), especially when the skin is wet after
perspiration (Roy et al., 2007).
Abrasion of the skin is necessary for removing the dead tissue of the skin and its protective oils,
which helps lower the electrical impedance and improve the quality of the collected signal (Tam and
Webster, 1977). A poor contact between the electrode and the skin would introduce additional noise,
so adhesive strips or tapes are usually used to secure the contact (Roy et al., 2007).
When a surface electrode is placed too far from the active motor units, the amplitude of the elec-
trical signal will be very low because of the quick attenuation throughout the tissue. The EMG signal
from deep muscles and adjacent muscles also interfere with the EMG of the muscle of interest, and
this interference is referred to as cross-talk. It is, therefore, necessary to put surface electrodes at a
proper position, where the cross-talk noise can be reduced and the EMG signal represents the muscle
of interest with a higher signal-to-noise ratio. There are established standards for placing electrodes
on different muscles (e.g., Zipp, 1982, for surface electrodes and Perotto and Delagi, 1994, for intra-
muscular recordings). The placement of surface electrodes can be easily performed by a person after
little training.
Percutaneous electrodes are invasive and can cause discomfort or pain, but they can be used to
measure the activity of deep muscles and the action potential of a single muscle fiber or motor unit.
Needle electrodes are inserted directly into the muscle of interest to measure the electrical activity.
Various needle electrodes are available for different measurement purposes, such as the monopolar
needle electrode, multipolar needle electrode, concentric needle electrode, and macroneedle electrode.
A needle electrode can be inserted into different parts of muscle, and the high selectivity allows it to
measure the MUAPT of individual motor unit or even the action potential of a single muscle fiber.
However, this high selectivity can also be a problem during the measurement of single-fiber EMG,
because the relative movement between the active muscle fiber and electrode may locate the pickup
area in a totally different muscle fiber and contaminate the signal. Therefore, care should be taken to
secure the needle position, especially during the measurement of single-fiber EMG.
Wire electrodes use very fine needles (~27 gauge) to insert the wires into the muscle of interest.
They are relatively painless compared to needle electrodes, and can be easily implanted and withdrawn.
Wire electrodes can also be used to measure the MUAPT of individual motor unit, and they generally
have a larger pickup area than that of needle electrodes so that they are not as selective as needle elec-
trodes. Nevertheless, fine wire electrodes may move within the muscle during a muscle contraction,
and this relative movement will influence the reliability of the measured signal.

12.2.3 Different Configurations of Electrodes
Electrodes can be classified into different configurations: monopolar, bipolar, tripolar, multipolar,
barrier and belly tendon electrodes (Loeb and Gans, 1986). Monopolar and bipolar electrodes are the
two main configurations used frequently.
In monopolar configuration, the electrical activity of a muscle is acquired by placing one detection
electrode in the pickup area, and another reference electrode in a place that is electrically quiet or a place
that has minimal physiological and anatomical association with the detection electrode. Therefore, a
monopolar electrode has a single recording point, and the signal is collected with respect to a remote
reference point. This configuration is often used in clinical applications because of its relative simplicity.
Monopolar surface electrodes detect a summation of all the electrical signals in the vicinity of the
detection surface and may introduce cross-talk noises from other adjacent or deep muscle tissues. This
leads to the further development of bipolar electrodes to counteract the cross-talk noise.
Bipolar electrodes have two detection electrodes at two recording points in the pickup area, and
another reference electrode is used at a remote position similar to that of monopolar configuration.