For electrode controlled prosthetic arms, commonly a one-muscle-for-one motion strategy is applied, that is, selecting a muscle site for control, and correlating it to a given actuator, to produce the desired motion. 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 movers.
This “ideal” myosite can be defined as that area where the electrode receives the maximum electromagnetic signals and where the electrode has good contact with the skin.
Electrode Positioning and Placement
Following a careful clinical examination, based on anatomical landmarks, the prosthetist has to identify the position of several forearms and arm muscles to be tested for site selection. To contract the required residual limb muscles for electrode testing, the patient is asked to perform different phantom motions. Surface electrodes are carefully placed over each individual muscle. The electrodes are placed over the agonist-antagonist muscle pairs to bring into effect the arm/forearm functions.
The accuracy with which the position of the electrodes has to be reproduced from one day to another to obtain the same functional patterns depends on the particular stump. The amputee is asked to contract and relax the muscles with the electrodes placed on the skin surface. The subjects are instructed to relax completely before and after each contraction.
Graphical recordings of the EMG signals picked up by the electrodes are made during each move. Occasionally, similar patterns for different motions are noted, necessitating the need to move the electrode to increase the signal on the electrode for that motion.
In the beginning, the prosthetist places the electrode in the center of the muscle bulk and notes the electromyographic activity. The electrode is then moved in different directions from the initial point, by half an electrode’s width. If the amplitude of the signal is greater in any of these new locations, the process is repeated using the new location as the starting point till the prosthetist locates the point with maximum amplitude. The different points are marked so as to create a border around the electrode site with the electrode site at its center.
When using multiple myosties, it is essential to locate isolated EMG signals, i.e. the signals that stay at a lower amplitude while the others are active and vice versa.
Precise electrode placement optimizes control for use of a bionic arm prosthesis. Distal myosites are preferable to allow adequate space within the prosthetic socket for electrode placement and good suspension. 2 myosites on the limb with the greatest difference in microvolt between them are identified. The therapist and prosthetist will identify the minimum signal necessary to operate the system
Factors for the selection of electrode sites for bionic armfitment –
- Volitional and consistent muscle contractions
- Not using the selected muscles to actively move the elbow.
- Electrode sites within the socket should not interfere with the socket fit.
- Good electrode to skin contact for the successful transfer of the EMG signal.
COMMON MYOSITES –
The muscle groups are typically used according to their physiologic function. Myosites that are commonly used to control the bionic arm include:
Shoulder Disarticulation – • TD closing, forearm pronation, and elbow extension: Pectoralis major muscle • TD opening, forearm supination, and elbow flexion: Infraspinatus or trapezius muscle
Transhumeral Level – • Closing and elbow extension: Biceps muscle • Opening and elbow flexion: 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 in the infraspinatus or trapezius posteriorly.
Transradial Level – • Closing the TD: Wrist flexors • Opening the TD: Wrist extensors