*2.3.1.1. Gelled EMG Electrodes*

Gelled EMG electrodes contain a gelled electrolytic substance as an interface between skin and electrodes. Oxidation and reduction reactions take place at the metal electrode junction. Silver – silver chloride (Ag-AgCl) is the most common composite for the metallic part of gelled electrodes. The AgCl layer allows current from the muscle to pass more freely across the junction between the electrolyte and the electrode. This introduces less electrical noise into the measurement, as compared with equivalent metallic electrodes (e.g. Ag). Due to this fact, Ag-AgCl electrodes are used in over 80% of surface EMG applications [10].

Disposable gelled EMG electrodes are most common; however, reusable gelled electrodes are also available. Special skin preparations and precautions such as (hair removal, proper gel concentration, prevention of sweat accumulation etc.) are required for gelled electrodes in order to acquire the best possible signal. Gelled EMG electrodes are shown in Figure 4.

#### **Figure 4.** Gelled EMG Electrodes

#### *2.3.1.2. Dry EMG electrodes*

Computational Intelligence in Electromyography Analysis – 430 A Perspective on Current Applications and Future Challenges

in Figure 3.

distorted.

**Figure 3.** Fine Wire EMG Electrode

**2.3. Surface EMG electrode** 

*2.3.1. Types of EMG Electrodes* 

*2.3.1.1. Gelled EMG Electrodes* 

that current can flow into the electrode.

prosthesis for physically disabled and amputated population.

inserted in the muscle throughout the duration of the test [5]. A fine wire electrode is shown

Surface EMG electrodes provide a non-invasive technique for measurement and detection of EMG signal. The theory behind these electrodes is that they form a chemical equilibrium between the detecting surface and the skin of the body through electrolytic conduction, so

These electrodes are simple and very easy to implement. Application of needle and fine wire electrodes require strict medical supervision and certification. Surface EMG electrodes require no such formalities. Surface EMG electrodes have found their use in motor behavior studies, neuromuscular recordings, sports medical evaluations [9] and for subjects who object to needle insertions such as children. Apart from all this, surface EMG is being increasingly used to detect muscle activity in order to control device extensions to achieve

Surface EMG has some limitations as well. Since these electrodes are applied on the skin, hence, they are generally used for superficial muscles only. Crosstalk from other muscles is a major problem. Their position must be kept stable with the skin; otherwise, the signal is

There are two types of surface EMG electrodes: Gelled and Dry EMG electrodes [10].

fact, Ag-AgCl electrodes are used in over 80% of surface EMG applications [10].

Gelled EMG electrodes contain a gelled electrolytic substance as an interface between skin and electrodes. Oxidation and reduction reactions take place at the metal electrode junction. Silver – silver chloride (Ag-AgCl) is the most common composite for the metallic part of gelled electrodes. The AgCl layer allows current from the muscle to pass more freely across the junction between the electrolyte and the electrode. This introduces less electrical noise into the measurement, as compared with equivalent metallic electrodes (e.g. Ag). Due to this Dry EMG electrodes do not require a gel interface between skin and the detecting surface. Bar electrodes and array electrodes are examples of dry electrodes. These electrodes may contain more than one detecting surface. In many examples, an in-house pre-amplification circuitry may also be employed in these electrodes. A reusable bar electrode is shown in Figure 5. Dry electrodes are usually heavier (>20g) as compared to gelled electrodes (<1g). This increased inertial mass can cause problems for electrode fixation; therefore, a material for stability of the electrode with the skin is required [10].

**Figure 5.** A Reusable Bar Electrode (an Example of Dry EMG Electrode)

### *2.3.2.Categories of Surface EMG Electrodes*

There are two categories of surface EMG electrodes [5]: Passive and Active EMG electrodes. They are briefly explained as follows:-

#### *2.3.2.1. Passive EMG electrodes*

These electrodes should be connected to an external amplification circuitry with the help of connecting wires for the proper acquisition of the EMG signal. Passive EMG electrodes can be disposable or reusable.

Electrodes shown in Figure 4 and Figure 5 both fall under passive surface EMG electrodes.

Signal Acquisition Using Surface EMG and Circuit Design Considerations for Robotic Prosthesis 433

reveals striking organization at the macroscopic level. The functional properties of the

i. Muscles with fibers that extend parallel to the muscle force-generating axis are termed **parallel, fusiform** or **longitudinally** arranged muscles. Examples of such types of

ii. Muscles with fibers that are oriented at a single angle relative to the force generating axis are termed **unipennate** muscles. Example of unipennate muscle is Extensor

iii. The angle between the fiber and the force-generating axis generally varies from 0° to 30°. The muscles are oriented at more than one angle. Most muscles fall into this category and they are called as **multipennate** muscles. Examples are Rectus Femoris

iv. The muscles which surround an opening so as to form a closed shape are known as **circular** muscles. Example of such kind of muscle is Orbicularis Oris (mouth muscle). v. The muscles in which their fibers converge on the insertion to maximize force of

There are various kinds of muscle fiber arrangements, which are discussed as follows:-

muscles are Biceps Brachii (bicep muscle) and Sartorius (groin muscle).

contraction are known as **Convergent** muscles. E.g. Pectoralis Major.

A detailed depiction of these muscle arrangements is provided in Figure 7.

skeletal muscle depend strongly on their architecture [12].

which is bipennate and Deltoid which is multipennate.

Digitorum Longus.

**Figure 7.** Muscles and their Architecture [13]

#### *2.3.2.2. Active EMG electrodes*

Active EMG electrodes contain a pre-amplifier attachment for surface electrodes. Needle and fine wire surface electrodes are also available. These electrodes usually fall under the dry surface EMG electrodes type. The in-house high impedance amplifier in these electrodes transfers the pre-amplified signal to the rest of the circuitry. Figure 6 shows an active EMG electrode.
