During muscular contractions in the human body, electrical currents are generated in the muscles representing the muscles’ neuromuscular activities. These signals produce ‘myoelectricity’ that is represented in the form of electromyographic signals. Bionic prosthetic arms consist of electrodes that pick up these electromyographic signals from the surface of the amputee’s skin. The sites or locations where the electrodes are placed are known as myosites. These electrodes are, therefore, placed on the myosite that has the maximum signal strength as well as good skin-to-electrode contact, otherwise referred to as an ideal myosite. Before placing the electrodes on the amputee’s limb, a prosthetist will clinically test for signal strength to determine which site has the maximum strength. This process is known as myosite testing or control site testing.
Myosite testing is done using an electromyogram or various other myosite testing tools, called myotesters that test the muscle signal strength when the patient performs a particular body movement. These myotesters have 2 active electrodes that are placed on the surface of the patient’s residual limb along with a ground electrode that the amputee holds in his/her sound hand. The myotester also has an adjustable dial that indicates the electrical potential of the different muscles.
PROCESS OF MYOSITE TESTING
Placement of electrodes
Following a comprehensive assessment and clinical examination, the prosthetist must first identify the location of the different forearm and arm muscles to be tested for the selection of myosites. The prosthetic arm system consists of surface electrodes that detect the electrical signals on the amputee’s limb. These electrodes are placed over each muscle and the patient is asked to perform different motions. The electrodes are then placed over opposite muscles, a pair of muscles that perform the opposing movements, also known as an agonist-antagonist muscle pair. For muscles that have been amputated, the patient is asked to perform phantom movements to allow the electrodes to pick up signals from the remaining muscles. With the electrodes placed on the surface of the skin, when the amputee relaxes and contracts his/her muscles, the electrode can detect and pick up the corresponding myoelectric signals from the muscles. The exact precision and accuracy with which an electrode on the particular myosite performs will vary from one day to another, as the patient undergoes physical therapy.
The electrode is first placed in the center of the muscle that is being tested. After taking note of the signal strength, the electrode is then moved to different directions to measure the single strength in the corresponding area. If the signal in these areas is more than that of the muscle center, these steps are repeated, using those new sites as the initial point. Once the myosite is decided, all the different points where the electrode was tested are marked in a pattern that creates a border around the myosite. Ideally, the electrode should then be placed in the center of this outlined area.
Optimal placement of electrodes on the myosites is very essential for the effective functioning of any bionic prosthetic arm. Precise electrode placement plays a vital role in the success of the bionic prosthetic arm, along with the appropriate training.
Some of the factors that affect the selection of myosite for electrode placement include:
- Good contact between the skin and the electrode to enable smooth transmission of the signal.
- Electrodes placed within the prosthetic socket must not interfere with the socket’s fit over the residual limb.
- Consistent and voluntary contraction of the target muscles.
Steps in myosite testing
Step 1 – Discussion between the patient and the prosthetist
A detailed discussion should be carried out between the patient and prosthetist to understand the process of electrode placement and myosite testing. The prosthetist will also determine the needs and current status of the patient through assessment and clinical evaluations.
Step 2 – Preparing the patient for myosite testing
The skin surface of the residual limb is lightly cleaned with an isopropyl alcohol swab to remove any oils that may be present on the skin. It is very important to remove any dirt/oil from the skin as they may interfere with the signal detection and myosite testing.
The prosthetist will then guide and demonstrate to the patient the process of muscle contraction and relaxation. While the patient relaxes and contracts the muscles of the residual limb (phantom movements), the prosthetist will palpate the different muscle groups to determine different sites for control.
Step 3 – Myotesting
Following the procedure mentioned above, the middle of the muscle belly is identified and marked. The skin of the patient’s residual limb is then moistened to improve electrical conductivity.
Myotesting starts by assessing a single muscle group at a time. The electrode gain dial is set on the back of the electrode to lower values, and then gradually increased to higher values. The electrode is placed over the marked locations established before and must run longitudinally with the muscle belly. The patient is instructed to contract his/her muscles, to produce open and close signals. If the individual is struggling, they can contract the same muscle(s) on the sound side (if applicable), to understand what the movement feels like.
Following this, the steps mentioned above are followed to establish a border for the electrode.
The muscle groups are typically used according to their physiologic function. Common myosites used to control the bionic prosthetic arm include:
Shoulder Disarticulation –
- For terminal device closing: forearm pronation, and elbow extension: Pectoralis major muscle
- For terminal device opening, forearm supination, and elbow flexion: Infraspinatus or trapezius muscle
Transhumeral Level –
- Closing of the terminal device and elbow flexion: Biceps muscle
- Opening of the terminal device and elbow extension: Triceps muscle
Biceps activity allows elbow flexion and hand closing, whereas triceps activity operates elbow extension and hand opening.
With short transhumeral amputations, myosites can often be located in the pectoralis or deltoid anteriorly and the infraspinatus or trapezius posteriorly.
Transradial Level –
- Closing the terminal device: Wrist flexors
- Opening the terminal device: Wrist extensors
Step 4 – Verifying Isolated Control
The patient is instructed to contract the “open” muscle group, relax, and then contract the “close” muscle group. While the patient repeats these muscle contractions, it must be checked that the opposite muscle group remains relaxed or does not overpower the active muscle group.
To ensure no unexpected motion of the bionic hand occurs when the patient moves their residual limb, the patient should be educated to relax all muscle signals while moving their residual limb in various planes of motion.
The patient should be able to activate their opening and closing muscle actions across various planes of motion to pick items up from different heights using their prosthetic arm.
For normal activities, often the muscles are in synergy such that individual muscle groups do not respond to isolation. Each body action requires the contraction and relaxation of multiple muscles, some quite distant from the body part being displaced. It has been noted that while performing a primary motion, different groups of muscles are activated for stabilization of the prime muscles.