BCIA Biofeedback Essential Skills: EMG

The surface electromyogram (EMG) is an indispensable tool for evaluating and training clients for chronic pain, headache, hypertension, neuromuscular disorders, optimal performance, and stress. We will cover seven topics in this post: sensor placement, color codes, skin preparation, impedance testing, factors that affect EMG values, bandpass selection, and normal values.

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Sensor Placement

Clinicians place active electrodes over the target muscle belly (i.e., central region) to record its full voltage. A stronger EMG signal enables an electromyograph to record muscle electrical activity more accurately. Explain muscle palpation and obtain written consent. With a staff member present, palpate the muscle with a gloved hand as it contracts and relaxes to find the muscle belly. Draw circles with a grease pencil to guide sensor placement. Space 0.5-cm active electrodes about 1.5 cm apart. Place the reference electrode within 15 cm of either active. Reference positioning is less critical than active placement.

In the graphic adapted from MYOWARE, compare the signal voltage from the belly with signals from distant sites. Electrodes placed near a muscle's insertion into a tendon or offset to the side reduce EMG signal strength. Likewise, placement over the wrong muscle or at an angle from the muscle belly results in misleading measurements (Sherman, 2002).

Thought Technology Ltd. generously provided the diagrams below from their Basics of Surface Electromyography Applied to Physical Rehabilitation and Biomechanics (2009).

EMG Sensor Color Codes

Note that the color code for active electrodes varies with the manufacturer. For Thought Technology Ltd., the active electrodes are blue and yellow.

For MindMedia, the active electrodes are white and red.

Skin Preparation

We recommend 14 site preparation steps: 1. Select the muscle site you want to monitor. 2. Explain how an EMG sensor operates, site preparation, and sensor attachment procedures. Obtain written permission to monitor your client. 3. Instruct your client to sit or stand. 4. While wearing a glove, palpate the muscle while contracted and relaxed to find the muscle belly. Position the active electrodes over the center of this region for the strongest signal. 5. Draw circles with a grease pencil to guide sensor placement. 6. Space 0.5-centimeter active electrodes about 1.5 centimeters apart. 7. Place the reference electrode within 6 inches of either active. 8. Gently wipe the skin with an alcohol preparation pad to remove oil and dirt. If needed. shave excess body hair.

9. Gently abrade the skin with a pad to remove dead skin. 10. Scoop up a small quantity of gel, like Nuprep, on a cotton swab and gently work it into the skin using small circular motions.

11. Apply EMG sensors to the skin. If the sensor has a floating cup design, apply the adhesive collar before filling the cup with conductive gel. Fill it completely and remove the excess with the collar's backing paper. If the sensor is disposable, press its pre-gelled surface to the skin.

12. Position the electrode cable to minimize discomfort and tension. 13. Tape the cable at several points for strain relief. 14. Instruct your client to minimize movement and confirm their compliance.

Impedance Testing

Measure the quality of skin-electrode contact, which is called impedance. An impedance meter, which sends a nonpolarizing AC signal through the skin, provides the most valid impedance measurement.

In bipolar recording, we measure the impedance between each active electrode and the shared reference. Impedance testing results in two measurements in the Kohm (thousand-ohm) range. An impedance test can be performed manually with a separate impedance meter or voltohmmeter or automatically.