What is myoelectric control

Myoelectric arm prostheses

Table of Contents







1 Introduction

Myoelectric prostheses are artificial limbs for people with lower or upper arm amputations, which partially replace the functionality of their lost arm or hand.

Compared to the lower extremities, replacing the upper extremities is associated with considerably more difficulties. On the one hand, the number of movement patterns is greater, and on the other hand, the individual prosthesis function such as grasping or elbow movements must have the necessary energy. The butt and mass forces are not sufficient for these functions. Nowadays the movements of the prosthesis are no longer generated directly by muscle power, but only controlled by the muscle. Depending on whether the body's own energy sources, i.e. remaining muscles or external energy sources, e.g. battery with motor, are used to operate the prosthesis, a distinction is made between body-powered and external-powered prostheses. Myoelectric prostheses, which are part of the external force prosthesis, use the electrical potential of the remaining muscles of the residual limb to control the prosthetic functions.

A finished prosthesis consists of the inner socket (residual limb bedding), the outer socket (shaping shell) and the system components such as system electric hand, elbow fitting, battery, etc.

In 1980 the term "bionic arm" was used for the first time. The term “bionics” is derived from “biology” and “technology” in the German-speaking world. New lightweight construction methods, more powerful components, microprocessors and new methods of signal processing enable what was not considered possible 20 years ago.1 This is where myoelectric prostheses are essential. In the future, they should offer additional options for movement, be able to be controlled as intuitively as possible and have natural, static and dynamic cosmetics.

The patient surveys are particularly important for the further development of myoelectric prostheses. These resulted in:

- "faster gripping speed,
- more natural shapes, shades and movements of the arm prostheses,
- lighter weight,
- larger capacity of the batteries,
- a dirt-repellent, easy-to-clean cosmetic protective glove,
- the force with which the arm prosthesis grips should be felt and
- the arm prosthesis should allow additional movements and types of grip. "22. Differentiation of myoelectric prostheses from other types of prostheses

In order to present various types of prostheses, this paper exclusively refers to the Otto Bock company. Which prosthesis is suitable for which patient and when is determined as part of a condition assessment in which the specific requirements and individual circumstances of a patient are checked. The condition assessment forms the basis for further individual care.

The prosthesis supply at Otto Bock is divided into three basic types:

2.1 Cosmetic arm prosthesis

Cosmetic, so-called passive arm prostheses are worn by people for whom the external appearance is of great importance. In addition to its purely aesthetic function, this prosthesis also helps in everyday life. Objects can be supported with it and used as a counter-hold for certain activities. Because it does not have active control elements, it has a particularly low weight, which is of great importance in the case of high amputation heights.

Figure not included in this excerpt

Figure 1: cosmetic arm prosthesis

Source: www.gluaserag.ch3

2.2 Pull-operated arm prostheses

Pull-operated arm prostheses are self-powered prostheses. The function of the prosthesis is controlled by the body's own forces, e.g. the stump and / or the shoulder girdle. The movement of the prosthesis is initiated via a power bandage, which usually runs from the prosthetic arm over the back to the loop around the healthy shoulder. The patient gets a feeling for the movement through the power transmission bandage by means of his own strength.

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Figure 2: Pull-actuated arm prosthesis

Source: www.gluaserag.ch4

Advantage: - simple mechanics

- favorable acquisition costs
- suitable for people who work in the water

Disadvantage: - unnatural movements

- Power bandages that take getting used to
- severely limited grip strength

2.3 Myoelectric arm prostheses

Myoelectric arm prostheses are external force prostheses, which means that they are not driven by the patient's muscle strength, but with the help of electrical energy. These prostheses are often called EMG (electromyographic) prostheses.

Figure not included in this excerpt

Figure 3: Myoelectric arm prosthesis

Source: www.gluaserag.ch5

Advantage: - suitable for people who cannot control a self-powered prosthesis or


- high grip strength with little intrinsic strength
- Minimization of non-physiological movements

Disadvantage: - heavy weight

- High acquisition costs
- cannot be used in damp rooms

3. Myoelectric prosthesis control

Two methods have been established in the past to control electrical prostheses.

Myoelectric prostheses are controlled with the help of electrodes that are in direct contact with the skin. Myoelectric prostheses are controlled via the electrical potential of the remaining muscles (flexor and extensor muscles). In contrast to this, pressure sensors or miniature switches are also used in other external force prostheses to control the active components. The electrodes are non-invasive and work reliably. In the event of a defect, the electrodes can easily be replaced. However, for each joint that is to be actively moved, one muscle group is required that should be contractible at will and independently of other muscle groups. In practice, 1–2 electrodes have proven effective.6 Technically, prostheses with 6 control levels have recently been implemented.

Put simply, with every contraction of a muscle (including the remaining muscles after an amputation), biochemical processes produce an electrical voltage in the micro-volt range, which can be measured on the skin. The low voltage is amplified and passed on to the prosthesis as a control signal.7 The cocontraction of both muscles allows you to switch between two movements. In this way, the same muscles can be used to control further movements. This is the case with the Otto Bock prosthesis for rotating the wrist.8

3.1 Signal transmission

The stimulus transmission systems of the human body are based on chemical and electrical processes. The transmission of stimuli within nerve or muscle fibers takes place on the basis of action potentials (electrical transmission). The action potentials of the muscle fibers serve as signal transmitters. The potential difference between the current size of the action potential and the resting potential of a muscle cell is called myoelectric voltage. However, the measured sum signal is called myoelectric voltage, since the tensing of a muscle is based on the contraction of several muscle cells and fibers. The measurement takes place superficially or intramuscularly using needle electrodes. Myoelectric signals can be measured directly intramuscularly, whereby surface recordings lead to signals of inferior quality. Here only a sum signal from several muscle fibers and / or muscle groups is measured. The tissue between the muscle and the sensor acts as a filter for high frequency components. The superficial scanning limits the evaluable frequency range of myoelectric signals to approx. 10 to 1000 Hz.9 If there are lesions of torn muscle groups, the signal quality deteriorates and important information for the interpretation of the signals is lost. However, according to Reischl (2006), studies with subcutaneous or implanted electrodes on the short amputation stump in the control of prostheses have not shown any significant advantages over surface sensors. A major and not insignificant disadvantage of needle electrodes is the increased risk of infection.

In order to be able to use surface EMG sensors, it requires the scanning of muscle groups close to the surface.

3.2 Signal recording

Man-machine interfaces (MMI) are used to evaluate and interpret conscious actions of the wearer.

“All perceptible components of a technical system (in short: machine) that are used for communication with their users are referred to as the human-machine interface. The necessary functions are to be distributed between humans and machines according to ergonomic criteria. "10

With myoelectric prostheses, especially with functional hand prostheses, MMI converts the contraction of the arm stump muscles of the patient / user into a movement of the prosthesis. This is done according to a predetermined scheme contained in the MMI. The wearer of an EMG prosthesis must be able to consciously or unconsciously contract his stump muscles in order to enable the MMI to carry out a uniform evaluation. The diagram for communication between man and machine is shown in Figure 4.

The human-machine interface records human actions in the form of sensor signals, which are obtained from the contractions of the muscles of the remaining arm stump, and makes them available to the machine in the form of control signals. The conversion of the sensor signals into control signals takes place through a bio-signal analysis in which the recorded signals are evaluated and interpreted.


1 see Pylatiuk (2006)

2 Quote from: Bionic arm prostheses by C. Pylatiuk, magazine: Der Orthopäde, volume 35, number 11 / November 2006, pages 1169-1175, Springer Verlag / Heidelberg

3 Source: http://www.glauserag.ch/produkte/prothesen.html?untergruppe=86&titel=Arm-Prothesen, download: 02/01/2009

4 Source: http://www.glauserag.ch/produkte/prothesen.html?untergruppe=86&titel=Arm-Prothesen, download: 02/01/2009

5 Source: http://www.glauserag.ch/produkte/prothesen.html?untergruppe=86&titel=Arm-Prothesen, download: 02/01/2009

6 see Pylatiuk (2006)

7 see Bock-DynamicArm®

8 see Reischl (2006)

9 see Reischl (2006)

10 Quote from: A method for the automatic design of human-machine interfaces using the example of myo-electric hand prostheses by Markus Reischl, dissertation 2005, University of Karlsruhe

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