P08027: Wireless Assistive Control System
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EMG

Table of Contents

Electromyography (EMG) is a process of recording muscle activity.

Bio-Signals

Electromyography is a process to record muscle activity. As a muscle is made, ions (potassium, calcium, etc) in the muscle move in order to cause the muscle to clench. This process will change the voltage potential to a new level. Instrumentation is used to detect this change. In order to measure this change, it's possible to hook a speaker to the instrumentation or a device to view the waveform. Visual representations of electromyography can be recorded different ways. By using an oscilloscope, the recording is in terms of voltage versus time. The level of the voltage is related to the number of action potentials that are being fired.

In order to measure an action potential, a transducer is required to provide a form of feedback. Biopotential electrodes convert between ionic current and electron current. Ion flow mimics electron flow. The ions in the body that primarily affect EMGs are potassium, calcium and sodium. As more muscle fibers are recruited to make a muscle, more ions flow which affects the resting membrane potential. Through oxidation, a current will arise and the change in magnetic field will be recognized by the electrode. Using invasive electrodes allow for muscle analysis that is specific to a certain muscle, or part of a certain muscle. By using surface electrodes, it will pick up whatever it can from the change in the ionic membrane.

There are typically three electrodes used into to measure the EMG signal. This is a V positive line, a V negative line and a ground. The ground is used as a reference. Since the signal propagates down the muscle, the change in potential requires two points. The difference between these two points is then compared to the reference line.

Signal Acquisition

The next step is to use an amplifier, which has a gain depending on the circuity, but it's typically anywhere from one thousand to tens of thousands of volts per volt. The amplifiers used in this situation are designed to have a very large input impedance. To view the signal, an oscilloscope can be used. A signal can be saved and then analyzed on a PC. The Oscilloscope acts as a analog to digital converter in this situation.

In order to process the signal, it's typically necessary to send the signal to a PC or a DSP. If a PC is used, then a analog to digital converter needs to be used. After the A/D converter, a program such as Matlab or LabView can be used to process the signal. The figure below illustrates a simple system to see and process a signal.

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Static vs Dynamic Movement

When recording EMGs, there are two typical classes of movement: static and dynamic. Static movement is when a weight is applied against a muscle, but the body is not moving. Dynamic movement consists of moving part of the body. The image below shows the difference between an EMG of a dynamic recording and a static recording over time.

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The static recording is from someone holding a book in place while the dynamic recording is from someone who flexed at the elbow in order to bring their hand to their shoulder (a bicep curl). The static recording appears cleaner and depending on the application, much easier to work with. When performing the action for the dynamic recording, it's possible for the electrode to pick up movement artifact. This is unwanted noise on the line. It can be demonstrated by shaking the EMG cables without using the muscle currently being examined.

Protection Concerns

When recording any bio-signal, it is paramount that the user is not harmed. Due to how electricity is transferred in the United States, a safety circuit is required. This is because AC current is being transferred at 60 Hz. Anything in the 50-60 Hz range is ideal for causing fibrillation. This can be combated several ways. One way is to use a medical isolation transformer. This transformer will prevent current spikes so that signals can be safely sent into and retrieved from the human body. Another method is to use a DC source. While a DC source can run off of an isolation transformer, it's best to use batteries which are inherently limited due to the power they can supply.

Conductivity Concerns

A source of poor conductivity could be due to the cleanliness of the person's skin. By using a clean wipe (with isopropyl alcohol), most skin oil and debris can be removed from the surface. The electrodes contain a conductive gel to help fill any small gaps between the actual electrode and the skin surface. It is possible body lotion can affect results based upon how much is present and whether or not it is conductive. Body hair can still be an issue. For some medical applications, it's best to shave the area. For P08027, alcohol prep pads will be used to clean the site for the electrodes, although it's possible to put the electrodes on without this step.